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POPULAR SCIEXCE. 




THE PLANET SATURN. 

5 VIEWER BY THE EKV. W. H. U*WKS, ON NOV. 29. 1850, SIIOV 




OP THB 8UN, A8 SKKN BY T 



TOTAL ECLIP3E 

-- - • -Sr^R ENGELHOLM. IN ^WEPffV. .Taf.V VS, 1^ 



THE SOLAR SYSTEM: 



A DESCRIPTIVE TREATISE 



UPON THE SUN, MOON, AND PLANETS, 



INCLUDING 



M Slrrnunt nf nil tjie %tmi Wmmnim 




BY J. EUSSELL HIND, 

FOREIGN SECRETARY OF THE ROYAL ASTRONOMICAL SOCIETY OF LONDON, 

CORRESPONDING MEMBER OF THE NATIONAL INSTITUTE OF FRANCE, 

AND OF THE PHILOMATHIC SOCIETY OF PARIS, ETC., ETC., AND 

FORMERLY OF THE ROYAL OBSERVATORY, GREENWICH. 



NEW YORK: 
GEO. P. PUTNAM, 155 BROADWAY. 

1852. 



J-o 



s*.. 



TO 

DR. MARSHALL HALL, 

FELLOW or THE EOTAL COLLEGE OF PHTSICIAXS, AXD OF 

THE EOYAL SOCIETY, AXD OF THE EOYAL SOCIETY OF 

EDIXBrEGH, FOEEIGX ASSOCIATE OF THE XATIOXAL 

ACADEMY OF MEDICIXE OF FEAXCE, ETC., ETC., 

AXD THE DISCOTEEEE OF THE SPIXAL SYSTEM IX PHYSIOLOGY, 

®l)iQ treatise 

IS IXSCEIBED, AS A SLIGHT TOKEX 

OF GEATITUDE AXD ESTEEM, 

AXD OF ADMIEATIOX OF HIS SCIEXTIFIC LAB0E3, 

BY 

THE AUTHOE. 



PREFACE. 



THE present work differs in its arrangement and general con- 

tents from any exclusively astronomical treatise with which 
I am acquainted. I have had in view the production of a descrip- 
tive work, presenting the reader with the latest information on 
all points connected with the Solar System, yet written in a style 
as popular as the nature of the subject will admit. It will, there- 
fore, be understood that this little volume has no pretences to 
the character of an explanatory treatise on astronomy, hut is 
rather addressed to that numerous class of readers whose time 
and inclination do not permit of any regular study of the princi- 
ples of the science, but are yet desirous of informing themselves 
as to the present state of our knowledge of the heavenly bodies, 
.what has already been accomplished, and how much there yet 
remains to be done. 

I have thought it necessary, however, to introduce frequent 
explanatory remarks, for the more ready comprehension of those 
parts of the work, which, without such additions, might appear 
obscure or unintelligible. 

The present treatise is confined to the Sun^ Moon^ and Planets; 
but, if life and health be spared me, I hope to carry out the same 
plan to Cometary and Meteoric Astronomy^ and also to the Stars 
and Nebul(B. The subjects are all so widely different, that it is 
no disadvantage to treat of them in separate works. 

To M. Le Yerrier, and to those English Astronomers who 
have kindly furnished me with more definite information on cer- 
tain points connected with their investigations than was to be 
found in printed authorities, I have to return my best thanks. 

J. RUSSELL HIIsrD. 

Grove Road, St. John's Wood, London, 
December^ 1851, 



CONTENTS. 



CHAPTKR PAGE 

I. THE SUX, 11 

II. THE IXFEEIOR PLANETS. MEECUEY, .... 23 

III. YENUS, 33 

lY. THE EAETH, 45 

Y. THE MOON, ........ 56 

YI. ECLIPSES OF THE SUN AND MOON, .... 85 

YII. THE SUPEEIOE PLANETS. MAES, . . . . 107 

YIII. THE MINOR OR ITLTEA-ZODIACAL PLANETS, . . .112 

CERES, 113; PALLAS, 115; JTNO, 117; YESTA, 118; 

ASTR^A, 120; HEBE, 121; IRIS, 122; flora, 124; 
METIS, 125; HTGEiA, 126; PARTHENOPE, 127; YIC- 

TORIA, 127; EGERIA, 128 ; IRENE, 129 ; EUNOMIA, 130. 

IX. jrpiTER, 132 

X. SATURN, 144 

XI. URANUS, 165 

Xn. NEPTUNE, 175 



THE SOLAR SYSTEM: 

OR 

THE SUN, MOON, AND PLANETS. 
CHAPTEE I. 

THE SUX. O 

npHE Sun, as the great originator of light and heat, and the 
-*- mighty centre of the system, first claims our attention. 

The distance of this splendid luminary from the earth, 
which is employed by astronomers as a common unit of meas- 
m-ement, has been ascertained with very great accuracy from 
the transit of Venus over the Sun's disc in 1 769. It will readily 
be imasrined that an exact knowled^'e of this distance is of hio^h 

o o o 

importance in various astronomical investigations, and it has 
accordingly formed the subject of several elaborate inquiries. 
Professor Encke, of Berlin, has produced a masterly treatise on 
the results to be deduced from the transit of 1769 ; he con- 
cludes that at the mean distance of the earth from the Sun, 
the equatorial semi-diameter of our globe would subtend an 
angle of 8'\5776,^ which is called the equatorial horizontal 

* Since the above was wiltten, Mr. Adams has drawn my attention 
to a remark of Professor Encke's, in his Astronomical Jahrbuch for 
1852; from which it appears, that in order to satisfy the observations 
of the transit of Venus by Pere Hell, in 1769, a small correction is 



12 THE SOLAR SYSTEM. 

parallax of the Suu ; hence we infer by trigonometry, that this 
himinary is separated from us by 24,047 times the earth's 
equatorial radius, or, more exactly, 95,298,260 Enghsh miles. 
And this is the most probable value that we are able to derive 
from existing data, though it is possible future observations may 
furnish a result with greater pretensions to accuracy. It may 
be safely asserted, that we know the true distance of the earth 
from the Sun, within the 300th part of the w^hole ; a most sat- 
isfactory conclusion, considering the magnitude and importance 
of the question. 

Knowing the mean distance of the Earth from the Sun in 
semi-diameters of our globe, it is easy to determine the real 
diameter of the solar orb referred to the same unit of measure- 
ment. The best and latest observations prove that when the 
Sun is at his mean distance from us, the diameter subtends an 
angle of 32' 0^'; and there appears to be little, if any, appre- 
ciable difference betw^een the diameters measured in a vertical 
and horizontal direction. Hence we conclude that the true 
diameter of the Sun exceeds the equatorial radius of the earth 
223.83 times, or measures 887,076 miles. This enormous globe 
has, therefore, 1,401,910 times the volume of the earth, and 
the mass is found to be upwards of 355,000 times greater. 

The appearance of the Sun, with the aid of telescopes, may 
be briefly described. When we examine his disc through the 
intervention of a dark glass, we perceive upon it black spots, 
or maculae^ surrounded by a lighter shade, or penumbra, w^hich 
in most cases has a similar form to the inclosed spot, though 

necessary to the above value of the Equatorial Horizontal Parallax. 
It is, however, so small, that it has not been thought necessary, or even 
advisable, to recompute the various distances of planets from the 
Sun, &c., given in this work. Professor Encke's result for the Paral- 
lax is confirmed in a remarkable manner by the independent research- 
es of a Spanish Astronomer, Don Jose de Ferrer. — Author. 



THE SUN. 13 

this does not invariablj happen, several dark spots being occa- 
sionally included in a common penumbra. Generally they are 
confined to zones, extending 35° on each side of the Solar 
Equator, leaving an intermediate belt where they appear much 
more rarely. They have now and then been noticed in higher 
latitudes ; but these instances may be considered as forming 
exceptions to the general rule. The solar spots are not perma- 
nent, they change their form from day to day, or even from 
hour to hour, sometimes vanishing in an incredibly short space 
of time, while others make their appearance as suddenly. The 
dark, or central part of the spot, disappears first, and the penum- 
bra gradually closes in upon it. When a spot is observed for 
any length of time, it is found to change its apparent position 
on the Sun's surface, becoming visible at first upon the eastern 
side, and in somewhat less than a fortnight disappearing near 
the western limb, while after the lapse of another like period, 
if it remain as before, it will re-appear upon the eastern limb, 
and again traverse the disc. To account for these phenomena, 
it is necessary to admit that the Sun rotates upon his axis in a 
direction similar to that of the diurnal revolution of the earth, 
or from west to east. 

Tobias Mayer records the appearance of a black spot upon 
the Sun on March 15, 1758, the diameter of which was one 
twentieth of that of the Sun. Sir W. Herschel saw one on 
the 19th of April, 17V9, sufficiently large and well-defined to 
be \isible to the naked eye. More recently M. Schwabe, of 
Dessau, who has paid much attention to solar phenomena, has 
observed several spots without the aid of a telescope. One 
visible in June, 1843, measured 167/' in breadth, and was seen 
with the naked eye for a whole week. As one second of arc 
on the Sun's surface includes a breadth of 460 miles, we infer 
that the spot viewed by M. Schwabe must have occupied a 
space 77,000 miles in diameter, or ten times gi*eater than that 



14 THE SOLAR SYSTEM. 

of the Earth. A group of spots with the penumbra surround- 
ing it, will frequently cover a much larger portion of the Sun's 
disc. One noticed in April, 1845, measured b' 20^', and 
another on the 6th of December, of the same year, was nearly 
of equal length. A cluster of spots seen at the Cape of Good 
Hope by Sir John Herschel, at the end of March, 1837, covered 
an area of nearly five square minutes, a space which the reader 
will duly appreciate on remembenng that the diameter of the 
Sun is only thirty-two minutes. A minute in linear dimension 
on his disc being 27,500 English miles, and a square minute 
756,000,000. Sir John Herschel observes, that we have an 
area of 3,780,000,000 miles included in one vast region of dis- 
turbance on this occasion. M. Schmidt, of Bonn, counted up- 
wards of two hundred single spots and points in one of these 
large groups visible on the 26th of April, 1846, and one hundred 
and eighty in another cluster in August of the preceding year. 
It has been found by continual observation of the spots 
that their number varies considerably in ditFerent years. It 
will sometimes happen on every clear day during a particular 
year, the Sun's disc always contains one or more of them, while, 
in another year, for weeks or even months together, no spots of 
any kind can be perceived. M. Schwabe, after twenty-five 
years close attention to the appearance of the Sun's surface, 
thinks he has discovered something like regularity in the preva- 
lence or otherwise of these phenomena, and is induced to sup- 
pose that the period of variation in the number is not far from 
ten years. It is not easy to imagine any adequate cause for 
this cyclical appearance of the spots ; but in the present state 
of astronomy it is unsafe to reject any of the indications of 
careful observation, simply because w^e cannot fully account for 
them."^ We would particularly recommend the solar phenom- 

*M. Schwabe has furnished a table exhibiting th^uumber of days 
in each year between 1826 and 1843 on which the sun was free from 



THE SUN. 



15 



ena to the attention of amateur astronomers, who with ordi- 
nary telescopes may do good service to the science by regularly 
watching and mapping down the spots day by day ; a work 
almost beyond the power of the professed obseiTcr, who has so 
many other claims upon his time and attention. 

Besides the dark spots already described, we remark upon 
the Sun's disc curved lines or streaks of light of a more lumi- 
nous character than the rest of the surface, which are generally 
found in the neighborhood of the black spots, or where they 
have previously existed ; not unfrequently the dark spots break 
out amongst them. These phenomena are termed faculce 
{lichtstreifen by the Germans), and are considered by Sir John 
Herschel as the ridges of immense waves in the luminous re- 
gions of the Sun's atmosphere, indicative of violent agitation 

spots, and the number of groups showed. This table is interesting 
in more than one point of view, and is here subjoined : — 



Year. 



1826 
1827 
1828 
1829 
1830 
1831 
1832 
1833 
1834 
1835 
1836 
1837 
1838 
1839 
1840 
1841 
1842 
1843 



Groups of spots 
observed. 



118 

161 

225 

199 

190 

149 

84 

33 

51 

173 

272 

333 

282 

162 

152 

102 

68 

34 



Days on which 

the Sun was free 

from spots. 


Number of ob- 
ser\ing days. 


22 


277 


2 


273 





282 





244 


1 


217 


3 


239 


49 


270 


139 


267 


120 


273 


18 


244 





200 





168 





202 





205 


3 


263 


15 


283 


64 


307 


149 


324 



10 THE SOLAR SYSTEM. 

in the neighborhood. Th^faculoe are not so generally noticed 
as the spots, possibly because they require much better optical 
means to show them well. Yet M. Schmidt says in the year 
1845 he never saw them at all, though during the early jjart 
of the year he used one of Fraunhofer's celebrated telescopes, 
of four feet focal length ; but on one day in 1844 they were 
unusually distinct and visible in considerable numbers. Care- 
ful examination, with proper optical aid, shows that the Sun's 
disc is covered with a fine mottled appearance, consisting of 
minute points, or, as Sir John Herschel terms them, " dark dots 
or pores," which are constantly undergoing some alteration. 
The appearance presented by this uniform mottling of the Sun's 
disc has been aptly compared by the same eminent astronomer 
to the " slow subsidence of some flocculent chemical precipitates 
in a transparent fluid, when viewed perpendicularly from above." 
The rotation of the Sun upon his axis was inferred, as al- 
ready remarked, from, observations on the positions of the spots 
upon his disc on successive days. Astronomers have differed a 
good deal in the periods they assign to this rotation ; still it is 
certain that we have now approximated within a very few hours 
of the truth.^ Perhaps the period assigned by M. Bianchini, 
from very careful measures, in the year 1817, may be taken as 

* We subjoin the times of the Sun's rotation, according to the va- 
rious astronomers, from the age of Cassini to the present day : — 

Cassini I. "by comparing his own observations with d. h. m. 
thoseof Scheiner, &c., . . . . . 25 14 5 

De La Hire, 25 8 56 

Lalande, 25 10 

Flauguergues, from observations in 1798, . 25 1 2 

Delambre, 25 17 

Mossotti, 25 10 13 

Taylor in 1835-6, 25 14 

Petersen, . 25 4 30 

Laugier, . . . . . . . 25 8 10 



THE SUN. 17 

one of the best results ; this gives 25d. Ih, 48m. for one side- 
real revolution upon the axis, agreeing closely with the more 
recent calculation of M. Laugier. Besides the time of rota- 
tion, observations of the solar spots enable us to ascertain the 
position of the equator, and its nodes in reference to the ecliptic 
or the great circle of the heavens in which the plane of the 
earth's path lies. According to the eminent mathematician and 
astronomer, M. Delambre, the angle between the solar equator 
and the ecliptic is 7^ 19', and the longitude of the node, or the 
point w^here the equator intersects the ecliptic is 80^ 45'. 
Some later observations by Dr. Petei-sen at Altona, assign 6^^ 
51' for the inclination, and 73^ 29' for the position of the node. 
There are difficulties in the way of an exact determination of 
these quantities, and not practical ones only, for some astrono- 
mers have strongly suspected that the spots really alter their 
position upon the Sun's disc, in which case the apparent diur- 
nal movement of the spots given by our observations will not 
be the real change due to axial rotation, but must be partly 
influenced by the proper motion of the spot itself. Hence 
probably arise the discordances which are apparent in the re- 
sults of different astronomers, and in the times of rotation de- 
duced by the same observer from observations of different spots. 
The Earth is in the line of nodes about the first weeks of 
June and December, and at these times the spots, in traversing 
the Sun's disc, appear to us to describe straight lines. As our 
globe recedes from the line of nodes, the apparent paths be- 
come more and more elliptical, until we have advanced through 
an arc of longitude of 90°, or arrived at our greatest heliocen- 
tric declination, when the ellipticity reaches its maximum, di- 
minishing again as we are carried forward to the other node. 
The paths of the solar spots consequently present the greatest 
curvature about the commencement of March and the middle 
of September. 



18 THE SOLAR SYSTEM, 

The discovery of the spots is usually dated about the begin- 
ning of the seventeenth century, or soon after telescopes came 
into use. It appears, from the papers of our countryman Har- 
riot, that he observed them on the 8th of December, 1610. 
Christopher Scheiner, Professor of Mathematics at Ingoldstadt, 
remarked them in March following, and pubhshed a voUimi- 
nous work upon the subject, entitled Rosa Ursina. The cele- 
brated Galileo noticed the spots about the same time, and, in a 
tract printed in 1613, he affirms that he had shown them to 
several persons at Eome in 1611, and had mentioned their ex- 
istence to other friends at Florence some months previous. 
John Fabricius observed them at Wittenburg about the same 
time as Scheiner, and gave an account of them in a small work 
published in June, 1611. All these discoveries w^ere very prob- 
ably entirely independent of each other ; but it seems quite 
certain that the first notice of a solar spot is to be dated at a 
much earlier period. Adelmus, a Benedictine monk, in a life 
of Charlemagne, mentions a black spot observed upon the Sun 
in the year 807, on the 16th of the calends of April or March 
lYth : this circumstance is recorded by many historians, includ- 
ing Bede, Polydorus Virgil, and Aimoin, monk of St. Germain 
de Pres. Averroes, a Spanish Moor, is reported to have ob- 
served dark spots upon the Sun's disc about the middle of the 
twelfth century. It has been suggested that the otherwise 
mysterious diminutions of the Sun's light when there was no 
eclipse, mentioned more than once by historians,* may have 
been owing to a great accumulation of spots upon his disc ; but 

* A remarkable instance is recorded by Keppler. Astronomies pars 
optica, in the following words — " Refert Gemma Pater et Filius.. anno 
1517 ante conflictum Caroli V. cum Saxoniae Duce, solem per tres dies 
sen sanguine perfusum comparuisse ut etiam Stellas plerseque in meri- 
die conspicerentur." The battle alluded to is that of Miihlberg, which 
was fought on the 24th of April, 1547. 



THE SUN. 19 

it certainly appears questionable whether they could congregate 
in such numbers as to materially lessen the intensity of the so- 
lar rays. 

A great number of opinions have been advanced with regard 
to the nature of the solar spots. Scheiner at first considered 
them to be solid bodies revolving round the Sun, and very near 
his surface ; in this opinion he was followed by Malapert, who 
termed them Sidera Austriaca Periheliaca ; by John Tarde, 
who, in his turn, called them Borhoina Sidera^ as having been 
discovered in the reign of Louis XIII. ; and by the capuchin 
Antonio de Rheita, who thought he had accounted for thefac- 
ulcBj or luminous spots, also, by supposing them to be owing to 
the intense hght reflected from the revolving planets on the 
Sun's surface. Galileo differed entirely from Scheiner and his 
followers, regarding the spots merely as clouds or exhalations 
from the Sun's surface, and urging as a fatal objection to Schei- 
ner's theory, that they are ever changing their form and gen- 
eral appearance, sometimes vanishing suddenly, and bursting 
forth again with equal rapidity in other places. The idea of 
their being solid bodies was therefore soon rejected. 

The opinion prevailing among the best authorities of the 
present day is, that these spots are portions of the dark body or 
surface of the Sun, which are occasionally rendered \asible from 
the temporary removal of the interposing luminous atmosphere, 
owing to local causes of disturbance, which, whatever be their 
true nature, appear to be predominant in the equatorial regions. 
Sir William Herschel has accounted for the penumbra, and 
general appearance of the spots, by supposing the existence of 
a transparent medium, which sustains the luminous atmosphere 
at a great altitude above the Sun's sohd dark body, " carrying 
on its upper surface a cloudy stratum, which, being strongly 
illuminated from above, reflects a considerable portion of the 
light to our eyes, and forms a penumbra, while the solid body, 



20 THE SOLAR SYSTEM. 

shaded by the clouds, reflects none." The disturbances which 
give rise to the visibility of the spots, Sir WilHam thinks to be 
due to powerful upward currents of the atmosphere. 

Before closing our account of solar phenomena, we must not 
omit a brief notice of the zodiacal light. In these high lati- 
tudes, it is not usually visible except about the months of March 
and April, in the evenings, after Sun-set, and September and 
October, in the mornings, before Sun-rise ; yet in some yeai*s it 
has exhibited itself in uncommon brilliancy as early as Janu- 
ary. In tropical climates, the zodiacal hght is far brighter, and 
more sharply defined than we ever see it in this country. Its 
appearance is that of a conical-shaped light, extending from the 
horizon nearly along the coui*se of the ecliptic, the vertex at- 
taining distances of ^iO^ or 80^ from the Sun's place, or, as 
some observations w^ould show, extending 100^ from the same 
point. Hence it is e^ndent its real extent must include the 
orbits both of Mercury and Venus, and possibly even that of 
the Earth. The visible length above the horizon, and the 
breadth of the hght at its base, vary under diflferent circum- 
stances, the latter from about 10^ to 30^. The general opin- 
ion is, that the axis of the zodiacal light is in the plane of the 
Sun's equator. M. Houzeau has endeavored to show, by cal- 
culation of a considerable number of observations by Cassini 
and others, that the elements of the zodiacal light are materially 
different from those of the Sun's equator : he fixes the node of 
the light in 2^ heliocentric longitude, subject to a probable error 
of 12° or 13^, and its inclination to the plane of the ecliptic 
3^.35^, subject to an uncertainty of rather more than 2^. With 
these elements he finds his series of sixty observations rather 
better represented than if the elements of the Sun's equator 
w^ere employed ; but the preference to be given to the former 
is by no means decided. The subject deserves fm-ther investi- 
gation, when a much larger number of observations are in our 



THE SUN. 21 

possession than that employed by M. Houzeau. At present, 
we think the evidence against the supposed coincidence of the 
above elements by no means sufficient to outweigh the proba- 
bilities in its favor derived from other considerations. Sir John 
Hei*schel suggests that the zodiacal light may be " no other 
than the denser part of the resisting medium,'' which, as we are 
now aware, has disturbed the movements of one, at least, of the 
periodical comets, " loaded perhaps with the actual materials 
of the tails of millions of those bodies of which they have been 
stripped in their successive perihelion passages ;" and the same 
eminent astronomer shows that it cannot be, as some persons 
have supposed, an atmosiDhere of tlie Sun, in the common ac- 
ceptation of the term, for dynamical reasons. 

In connection with Sir John Herschel's idea relative to the 
nature of the light, it is perhaps worthy of mention, that during. 
the visibility of the magnificent comet of March 1843, which 
exhibited a tail 50'^ long, and almost grazed the solar orb, the 
zodiacal hght was unusually brilhant — so much so, in fact, that 
some confusion was caused by the pubhcation of descriptions, 
of the latter phenomenon, which the observer appears to have 
mistaken for the coraetary train. 

On no recent occasion has the Light shown itself so conspic- 
uously or for so long a period, as during the early part of the 
year 1850. From the middle of January to the latter end of 
March it was constantly visible on clear evenings, but was 
brightest early in February, when it decidedly excelled the 
most condensed part of the Via Lactea about the constellation 
Cygnus. Observers who paid particular attention to the posi- 
tion of the bordei-s of the hght among the stars on this occasion, 
from pretty distant stations in England, have suspected the ex- 
istence of a very sensible parallax, but it is hardly necessary to 
remark that the apparent variation in the position of the out- 
line, as assigned at two distant places on the same evening, 



22 THE SOLAR SYSTEM. 

may be satisfactorily accounted for by the supposition of vary- 
ing atmospheric conditions. It is not possible to admit the re- 
ality of the parallax, if the luminosity observed in the western 
heavens in the early part of 1850, were, as we are at present 
under the necessity of regarding it, an appearance of the zodi- 
acal light. 

The first particular description of this phenomenon was 
given by Cassini the Elder in 1683, but it had been previously 
treated of by Descartes and Childrey, and it seems probable 
that it may have been remarked more than tv/o thousand years 
ago. 



CHAPTEE II. 

THE IXFERIOK PLANETS. 

VITHIN the orbit of the Earth re\'olve the two planets Mer- 
cury and Venus, recognized as such from the roost remote 
antiquity. We know that these bodies move in smaller orbits 
than our globe ; first, because they never appear in the opposite 
part of the heavens to that which the Sun occupies, or, to use 
astronomical language, never come into opposition with that 
luminary : secondly, because under telescopic aid they present 
every variety of phase from the thin crescent to the fully illu- 
minated disc, which should occur if, receiving their light from 
the Sun, they were always situated within the Earth's orbit ; 
and, thirdly, because at certain times we actually observe them 
projected upon the disc of the Sun in their passage between 
that body and our globe, and have watched them in their pas- 
sage over him ; a phenomenon known as a transit. 

MERCURY. ^ 

The first of the inferior planets is Mercury, who performs his 
revolution round the Sun in 87d. 23h. 15m. 43.9s. at a mean 
distance of 36,890,000 miles. When between the Earth and 
the Sun, or near the time of inferior conjunction^ the disc of this 
planet, as viewed from our globe, subtends an angle of about 
twelve seconds of arc, but the diameter dwindles down as Mer- 
cury approaches the opposite part of the orbit, where the breadth 
would not exceed five seconds. 



24 THE SOLAR SYSTEM. 

The constant proximity of the planet to the solar rays, has 
greatly interfered with observations of its physical appearance. 
The German astronomer, Schroter, who observed at the begin- 
ning of the present century and paid mnch attention to the sub- 
ject, considered he had decided evidence of the existence of 
high mountains on the surface of Mercury, and it was by exam- 
ining them at various times that he concluded the planet had a 
revolution upon its axis in 24h. 5m. 28s. ; but this inference 
may yet require very considerable modification. Sir W. Her- 
schel never remarked any spots upon the planet's surface, by 
which he could approximate to the time of rotation, nor are we 
aware that any astronomer since the time of Schroter has been 
able to add to our knowledge on these points. 

The eccentricity of the orbit of Mercury, or the deviation of 
his orbit from a circle, is much larger than in the case of any 
other of the old planets, and this circumstance, combined with 
the great inclination of his equator to the plane of his annual 
path, which Schroter thinks may amount to TO*^, must produce 
a vast variety of seasons, with great extremes of heat and cold. 
At perihelion, Mercury is only 29,305,000 miles from the Sun's 
centre, while in the opposite part of the orbit, or in aphelion, he 
reaches to 44,474,000, making a variation of distance arising 
from the eccentricity of his annual track, of no less than 
15,169,000 miles, which is nearly ^xe times as great as in the 
case of the Earth. 

The elongation or angular distance of Mercury from the Sun, 
measured as an arc of longitude, is never so great as 30^ : con- 
sequently he cannot be seen except in strong twilight, either 
morning or evening, and under the most favorable circumstances 
does not appear conspicuous to the naked eye, but twinkles hke 
a star of the third magnitude with a pale rosy light. We can- 
not, therefore, too highly appreciate the diligence and attention 
of the ancient astronomers, who were not only aware of the 



MERCURY, 25 

existence of the planet, but approximated very closely to his 
period, and were able to explain the general nature of his path 
in the heavens. Nevertheless we read of astronomers, and by 
no means inattentive ones either, who have lived and died with- 
out once seeing Mercury. Even Copernicus, the celebrated re- 
viver of the true system of the Universe, was never favored with 
a view of the planet, a circumstance attributed by Gassendi to 
the vapors prevailing near the horizon on the banks of the 
Vistula. 

On examining Mercury with telescopes of adequate power in 
different parts of his orbit, we notice phases similar to those 
presented by the Moon in the course of her revolution round 
the Earth, with which every one is familiar. At the greatest 
elongations eastward or westward we see only half the disc il- 
luminated, as in the case of our own satellite at first or last 
quarter. As he moves towards superior conjunction, his form 
becomes gibbous, and the breadth of the illuminated part in- 
creases, or the outline of the disc becomes more nearly circular 
the nearer he approaches that position. Owing to the intensity 
of the solar light we lose the planet for some little time previous 
and subsequent to the superior conjunction, but, on emergence 
from the Sun's rays, we find the form still gibbous, the gibbosity 
being now on the opposite side. The illuminated part dimin- 
ishes as the planet draws near its greatest elongation, about 
which time it is again seen as a half moon under telescopic aid ; 
and, as it advances towards inferior conjunction, the form be- 
comes more nearly that of a crescent, until it is lost for the 
second time in the Sun's refulgence, except at certain epochs of 
not very frequent occurrence when we see it as a black spot 
traversing his disc ; a phenomenon appropriately termed a 
Transit of Mercury, 

The real diameter of Mercury appears to be about 2950 
miles ; this value being deduced from very accurate measures 



'^ 



26 THE SOLAR SYSTEM. 

taken during the last few years. There is but little difference 
between the polar and equatorial diameters, the compression 
probably not exceeding 1-150. 

As ^ir as we are aware, Mercury is not attended by a satel- 
lite, and the determination of his mass, therefore, becomes a vejy 
difficult and uncertain matter. But it fortunately happens that 
we have a curious method of approximating to this element, 
viz., by the perturbations produced by the planet in the move- 
ments of a comet known as Encke's, which revolves round the 
Sun in little more than three years, and occasionally approaches 
very near Mercury about the times of perihelion passage. On 
this subject we shall have more to say when we come to treat 
of the comets. We shall here merely state the result thus ob- 
tained, which indicates that the mass of the Sun exceeds that 
of the planet 4,865,750 times, or the mass, as usually expressed, 
is 1-4,865, 750. The density of Mercury under this new mass 
is 1.12, that of the Earth being put equal to unity. 

In order that a transit of this planet over the disc of the Sun 
may take place, it is necessary that the Earth should be in the 
hne of nodes of Mercury at, or very near, the time of his pas- 
sage through them, this bringing the three bodies very nearly 
in the same line. Th^ nodes are situate at present in 46.7^ 
and 226.7^ of hehocentric longitude, at which points the Earth 
arrives about the 10th of iJ^ovember and the 7th of May, and 
in consequence of the very slow sidereal motion of the nodes 
(amounting to only 13' in one hundred years), the transits of 
Mercury must occur for a long time to come in one or other of 
these months, those at the Ascending Node taking place in 
November, and those at the Descending Node in May. 

The first recorded phenomenon of this kind occurred on the 
7th of November, 1631. In a dissertation published at Leipsic 
in 1629, Kepler notified to astronomers, that according to his 
calculations a transit must occur on this day, since at the time 



MERCURY. 27 

of conjunction he had found the latitude of Mercury by his 
tables less than the Sun's semi-diameter. This interesting pre- 
diction was verified by Gassendi, at Paris. He discovered the 
planet on the disc of the Sun shortly before nine o'clock in the 
morning. At first he thought it a spot which had not been re- 
marked on the preceding day, but continuing his observations, 
its motion was soon detected, and he saw the planet leave the 
Sun's disc on the western hmb about half-past ten, a.m. It 
was found that Kepler's tables represented the circumstances 
with far greater precision than even the author himself had 
hoped for. 

The second observation of a transit of Mercury was made 
by Jeremiah Shakerley, on the morning of the 3d of Novem- 
ber, 1651, at Surat, in the East Indies. It is said Shakerley 
was so desirous of witnessing the phenomenon that, hanng 
found by his calculations it would be invisible in England, he 
made the voyage to India for the purpose. 

The third recorded transit was observed at Dantzic by the 
celebrated astronomer, Hevelius, on the 3d of May, 1661. He 
saw the planet on the Sun's disc four houi^ and a half. 

The next transit took place on the 'Zth of November, 16'7'7, 
and was witnessed by our illustrious Halley, at St. Helena, and 
by M. Gallet, at Avignon. Halley thought the times of ingress 
and egress might be observed within a single second of time, 
and pointed out how the Sun's parallax might be ascertained 
from such observations, taken at places widely distant from 
one another, remarking, however, that the difierence of the par- 
allaxes would not be large enough to give very certain results. 
Accordingly, the transits of Mercury have not been employed 
for the above purjDose, but we shall presently have occasion to 
notice a similar phenomenon in the case of Yenus, which is far 
better adapted to give us a correct value of the Solar Parallax. 

A transit of Mercury occurred on November 10, 1690, and 



28 



THE SOLAR SYSTEM. 



was observ'ed in China by the Jesuit missionaries, at Erfurt by 
Godfrey Kirch, and at Nuremberg by Wurzelbaur. Another 
in 1697, on November 3, was witnessed by astronomers at 
Paris, and other places. One on the ninth of November, 1723, 
was watched at Paris, Genoa, Bologna and Padua, but the first 
complete European observation of a transit of Mercury bears 
date November 11, 1736, when nearly all the astronomers of 
the time observed the planet in its progress across the Sun. 
Since this epoch the phenomena have been pretty closely 
watched. The transit of 1802, November 9, was seen by the 
well-known Jerome de Lalande, who was the more interested 
in it, inasmuch as he remarks it was the last he could hope to 
witness. That of 1832, May 5, was visible in this country, 
though a general prevalence of unfavorable weather occasioned 
much disappointment. The next occurred on the 7th of No- 
vember, 1835, but was not visible in these parts of the Earth. 
Another on 1845, May 8, was partially observed in this country, 
and also the last, on November 8, 1848, which is the twenty- 
fifth that has occurred since the phenomenon was fii*st noted by 
Gassendi. 

The following table exhibits the circumstances under which 
the remaining transits of the present century will take place. 
The numbei-s have no pretensions to extreme accuracy, as the 
tables, both of the Sun and planet, have been considerably 
improved since the calculations were made by M. Lalande : — 



Year and Day. 


Greenwich Mean 

Time of 

Conjunction. 


Duration of 
Transit. 


Least Distance of 

Mercury from the 

Sun's centre. 




H. M. s. 


H. 


M. S. 


M. s. 


1861. November 11, 
1868. November 4, 
1878, May 6, . . . . 
1881, November 7, 
1891, May 9, . . . . 
1894, November 10, 


19 20 12 
18 43 44 

6 38 29 
12 37 37 
14 44 56 

6 27 4 


4 
3 
7 
5 
5 
5 


46 
30 42 
47 2 
18 18 

8 40 
15 12 


10 52 N. 

12 20 S. 

4 39 N. 

3 57 S. 
12 21 N. 

4 20 N. 



MERCURY. 29 

In describing their observations of the transits of Mercury, 
astronomers make use of the terms internal and external con- 
tacts. At the ingress or entrance of the planet upon the Sun's 
disc, the external contact takes place when the limb of the 
planet fii-st makes a perceptible indentation on the limb of the 
Sun ; this phenomenon can never be observed with any great de- 
gree of accuracy, and is therefore less important than the ob- 
servation of internal contact, or the moment when the whole 
disc of the planet is fairly projected on the Sun's surface. When 
a fine thread -of light is seen between the outer limb of the 
planet and the Sun's limb, the internal contact has passed. 
At the egress, or on the planet's leaving the solar disc, these 
contacts of coui^e recur, but in reversed order. The moment 
of internal contact is indicated by the disappearance of the 
thread of light, and that of external contact by the absence of 
all appearance of indentation or distortion of the Sun's limb. 

Before leaving this subject, we may notice several curious 
phenomena which have been remarked by astronomers during 
their observations of the Transits of Mercury. At the first ex- 
ternal contact, something like a penumbra or light shade upon 
the Sun's disc has been remarked. As the planet advances 
towards the internal contact, a " black drop" or line has ap- 
peared to connect its limb with that of the Sun, or the contour 
of the planet has been seen distorted in such a manner as to 
give it a pear-shaped form just before the formation of the 
luminous thread. Such appearances are doubtless to be as- 
cribed partly to atmospheric circumstances ; but this cause 
alone is not sufficient to account for them completely, for dif- 
ferent telescopes at the same station have frequently given very 
different results, the distortion of the outer hmb of Mercury 
being apparent in some instruments, while in othei*s nothing 
of the kind has been remarked. It happened thus at the last 
transit in November 1848, when an elongation of the planet's 



30 THE SOLAR SYSTEM. 

limb was distinctly seen with one telescope at the Royal Ob- 
servatory, though others afforded no indications of it. An- 
other singular appearance which has been mentioned by several 
observers at different transits, is that of a luminous sj)ot or 
" globule" upon the disc of the planet when projected upon 
the Sun. It was remarked by Wurzelbaur at Nuremberg, in 
November 1697 ; again at Utrecht, in May 1832, by Professor 
Moll, who says its periphery was not well defined, but seemed 
gradually to sink from a grayish white to the dark color of 
the planet's disc : it was always situated in the same position, 
or a little south — preceding the centre of Mercury. A teles- 
cope by DoUond and another by Fraunhofer showed the spot 
in precisely the same manner, though various eye-pieces, -mag- 
nifying from 96 to 324 times, were employed. A similar 
roundish spot of a grayish tinge was noticed at the last transit 
in 1848, in England and America. Luminous rings round the 
disc of the planet have been repeatedly noticed, and on other 
occasions dark or nebulous rings have been remarked. In 
1799 and 1832, the ring had a darker tinge of a violet hue, 
the color being strongest near the planet. These phenomena 
may probably arise from a simple cause, though at present it 
is very imperfectly understood. An American observer of the 
transit in 1848, says the dusky ring only appeared when the 
Sun was covered by a thin haze ; yet it is not improbable that 
the planet's atmosphere may cause a similar nebulous-looking 
ring. The " black drop" already mentioned as having been 
observed at the ingress^ is occasionally recorded at the egress, 
the hmb of Mercury being drawn toward that of the Sun, so 
as to cause a distortion in the opposite direction to that which 
is observed to take place at the planet's entrance upon the 
Sun's disc. 

Such are the most remarkable circumstances connected with 
the appearance of Mercury during his transits. 



MERCURY. 31 

The most ancient observation of tliis planet that lias de- 
scended to us is dated in the year of Nabonassar 494, or 60 
years after the death of Alexander the Great, on the morning 
of the 19th of the Egyptian month Thoth^ answering to the 
loth of November in the year 265 before the Christian era. 
The planet was observed to be distant from the right line join- 
ing the stars called ^ and d in Scorpio, one diameter of the 
Moon ; and from the star {^, tvv^o diameters towards the north, 
and following it in Right Ascension. Claudius Ptolemy re- 
ports this and many similar observations extending to the year 
184 of om^ era, in his great work known as the Almagest. 

We have also observations of the planet Mercury by the 
Chinese astronomers, as far back as the year a.d. 118. These 
observations consist, for the most part, of approximations 
of the planet to stars. M. Leverrier, the eminent French 
geometer, has tested many of these Chinese observations by 
the best modern tables of the movements of Mercury, and 
finds, in the greater number of cases, a very satisfactory agree- 
ment. Thus, on the 9th of June 118, the Chinese observed 
the planet near a cluster of stars in the constellation Cancer, 
usually termed Prsesepe ; the calculation from modern theory 
shows that on the evening of the day mentioned. Mercury was 
less than one degree distant from the group of stars. 

Although the extreme accuracy of observations at the pres- 
ent day renders it unnecessary to use these ancient positions of 
the planets in the determination of their orbits, they are still 
useful as a check upon our theory and calculations, and possess, 
moreover, a very hig^h degfree of interest on account of their 
remote antiquity. 

The tables of Mercury at present employed in the com- 
putation of Ephemerides, or for predicting the place of the 
planet at any time, as viewed from the Earth or Sun, are those 
of Baron Lindenau, pubhshed in 1813. T! ese tables etill 



32 THE SOLAR SYSTEM. 

agree, within moderate limits, with the results of observation ; 
but M. Leverrier has greatly improved the theory of the planet 
within the last few years ; and it is understood that tables 
based upon his more correct elements are in course of publica- 
tion. We have given the numbers assigned by this eminent 
geometer in the table of Planetary Elements, together with 
those which form the basis of the Baron Linden au's calcula- 
tions. 



CHAPTEE III. 

YENUS. ? 

THE second planet in order of distance from the Sun is 
Yenus, the most conspicuous of all the members of the 
planetary system, when she is favorably placed with respect to 
our globe. Her sidereal revolution round the Sun is performed 
in 224d. 16h. 49m. 8s., at a mean distance of 68,770,000 
miles, or nearly double that of Mercury. 

The apparent diameter of Yenus varies much more sensi- 
bly than that of Mercury, owing to the greater extent of varia- 
tion of distance from the Earth. At inferior conjunction, or 
near this point, her disc subtends an angle of about seventy 
seconds of arc, — while, at superior conjunction, it is less than 
ten seconds. The disc is never fully illuminated except at 
superior conjunction, when the planet is lost in the Sun's rays : 
but with a good telescope we may trace the variations in her 
form from the full gibbous to the narrow crescent, — changes 
which follow the same law as in the case of Mercury. 

The real diameter of the planet is not very accurately 
known ; the best observations assign about 7,900 miles, or 
about the same as the diameter of the Earth. We have at 
present no means of determining from actual observation the 
exact difference between the polar and equatorial diameters, 
but it is certainly very small. 

The disc of Yenus under telescopic vision is far too bright 
and glaring to allow of our obtaining any very precise knowl- 

2^ 



34 THE SOLAR SYSTEM. 

edge of the constitution of her surface. The elder Cassini 
watched the planet attentively about the year 1667, and on 
several occasions remarked ill-defined dusky spots, which he 
observed with the \iew of ascertaining the time of axial rota- 
tion. This he considered to be about 23h. 16m. 

An Italian astronomer, Bianchini, soon afterwards published 
some observations, from which he inferred that Venus occupied 
no less than twenty-four days in revolving upon her axis. So 
marked a disagreement in the conclusions of two observei*s, 
could not fail to attract the attention of Sir William Hei-schel, 
who carried on for many years a careful examination of the 
planet's surface, partly with the view of determining which of 
the periods was the correct one. He occasionally saw spots 
upon the disc of Venus, particularly in the summer of 1780, 
but the result of his observations would not give the time of 
rotation. ^' For," he observes, " the spots assumed often the 
appearance of optical deceptions, such as might arise from pris- 
matic affections, and I was always very unwilling to lay any 
stress upon the motion of spots, that either were extremely 
faint and changeable or whose situation could not be precisely 
ascertained." The great power and light of the forty-feet re- 
flector was found to be rather an inconvenience than otherwise 
in these observations ; in fact large telescopes of any kind have 
seldom been employed with advantage on the planet Venus. 
Sir William Herschel considered he had decisive evidence of 
the existence of a dense atmosphere, to the effects of which he 
attributed the appearance of a luminous border or bright mar- 
gin on the fully illuminated limb of the planet, from which the 
light diminished pretty suddenly. The prolongation of the 
cusps of Venus beyond a semicircle was also thought to be 
owing to the refraction in the atmosphere. The terminations 
of the cusps were always observed to be sharply defined and 
perfectly free from irregularities, such as the appearance of 



VENUS. 35 

mountains on the surface might occasion. Schroter, who paid 
much attention to his observations on this planet, assures us 
that mountains exist upon it of fifteen and even twenty miles 
altitude, or of far greater height than any upon the earth, and 
he remarked further that the greatest inequahties are in the 
Southern hemisphere.* The same astronomer, from closely 
watching the atmospheric spots and appearance of the horns, 
and from eight observations of a fixed point on the surface, as- 
certained that the time of rotation is 23h. 21m. 7.98s., a result 
which has been pretty generally received, though it may here- 
after be somewhat modified. In confirmation of this period of 
revolution, it may be remarked that Cassini II. was able to 
show the fallacy of Bianchini's inference by comparing all his 
father's observations together, and he further proved that the 
particulars given by Bianchini could be represented by a rota- 
tion of little less than one of our days.f Sir William Herschel 

* For a month following the 11th of December, 1789, Schroter 
noticed that the Southern horn appeared blunt with an enlightened 
mountain in the dark part of the disc, which was found to be 18.000 
toises in height, or rather less than twenty-two Enghsh miles. The 
highest mountain was supposed by Schroter to be 18.900 toises in 
altitude. These numbers, however, must be received with caution, 
for it may be doubted whether the micrometers, (S:c., employed by the 
diligent and able observer at Lilienthal, were sufficiently delicate for 
measures of this nature. His measures of the diameters of some of 
the minor planets are well known to be greatly in excess of the values 
given by the improved instruments of the present day. 

•f The same remark may probably apply to some observations on 
a spot upon the disc of Yenus, by M. Flaguergues, at Yiviers, between 
the 7th and 13th of July, 1796, which are said to favor Bianchini's 
conclusion with respect to the time of rotation. The observations in- 
dicated that the axis of Venus is inclined at an angle of 73° 32', to 
that of the elliptic (which agrees with Cassini's result), the North 
Pole being directed to 321° 20' longitude. These particulars were 
communicated by M. Flauguergues to the Academy of Sciences at 
Nismes, and are pretty satisfactory as regards the position of the axis. 



36 THE SOLAR SYSTEM. 

was of opinion that the time of revolution could not be so lono^ 
as twenty-four days, though, as above stated, his own ex- 
perience did not enable him to assign the precise period. The 
late Professor De Vico, in the admirable sky of Eome, fre- 
quently saw spots upon Venus ; but for a steady view of them 
it was necessary to wait for the very best atmospheric con- 
ditions, even under an Italian sky, as the author was assured 
by this dihgent observer. Professor Madler has recently made 
a series of observations, from which he deduces the amount of 
horizontal refraction, and finds it one sixth greater than in our 
own atmosphere. 

Venus is a morning star from inferior to superior conjunction, 
and an evening star from superior to inferior conjunction. Her 
greatest elongation from the Sun, in longitude, is about 4Y° 
15', hence she is never observable more than from three to four 
hours after sun-set or before sun-rise. Occasionally she attains 
so great a degree of brilliancy as to be distinguishable at noon- 
day in a favorable sky, without the assistance of a telescope.* 
This happens once in eight years, when the planet is at or near 
its greatest north latitude, and about five weeks from the time 
of inferior conjunction. One fourth of the disc, or rather less, 

* Claudianus relates that in the fourth year of Honorius, Emperor 
of the West, or a.d. 398, a star was seen in the day-time as bright as 
Arcturus appears at night. Venus might be observed at noon-day 
about the end of January and beginning of February in this year. It 
was probably this planet that attracted attention in the day-time in 
984, according to a Saxon Chronicle, and again on Easter Sunday, 
1008, and at the end of the year 1014. Several writers mention the 
appearance of a star at the sixth hour of the day, or about noon, on 
Palm Sunday 1077 (April 9) ; Venus was then approaching her inferi- 
or conjunction, and might probably be the object here referred to. 
On the 29th of January, 1280, she was seen by day-light, and again 
/or some days about the 27th of May, 1363, like a very small star. 
Very recently the curiosity of the Parisian public was excited by the 
discovery of a star in the day-time, which proved to be Venus. 



VENUS. 37 

is illuminated, and under these circumstances the planet has 
been observed to cast a very sensible shadow at night. The 
elongation from the Sun at the time of maximum brilhancy is 
rather less than 40^, and the diameter of the illuminated part 
about ten seconds of arc ; the phase is therefore similar to that 
presented by our own satellite about three days fi-om the Xew 
Moon. 

Astronomei's are by no means satisfied whether the planet 
Venus be attended by a satellite or not. Observations have 
been made which are strongly in favor of the existence of such 
an attendant, but many of the most diligent observei-s of past 
and present times have watched the planet under every variety 
of climate and atmospheric condition and with telescopes of all 
kinds, without once obtaining a glimpse of a satellite. It is a 
question of great interest, and must remain open for future de- 
cision. "We shall here briefly recapitulate the evidence in favor 
of a satellite. 

The celebrated Cassini was the fii'st astronomer who noticed 
any suspicious object near the planet On the mornings of 
January 25th, 1672, and August 27th, 1686, he distinctly per- 
ceived a luminous body, presenting on the first occasion the 
same phase as Venus, and about one quarter of her diameter. 
The well known optician. Short, remarked an object with the 
same phase as the planet, and about ten minutes of space from 
it, on the morning of the 3d of ISTovember, 1740. In the 
month of May, 1761, M. Montaigne, at Limoges, saw what he 
considered to be a sateUite on four evenings. It was always 
one fourth of the diameter of Venus, with precisely the same 
form, and changed its position with respect to the planet. In 
March, 1764, the supposed satellite was observed by several 
astronomei-s, and, what is most important, at places widely dis- 
tant from one another. Rodkier, Horrebow, and others at 
Copenhagen, with a refracting telescope, and Montbarron at 



38 THE SOLAR SYSTEM. 

Auxerre, with a Gregorian reflector, repeatedly saw the attend- 
ant between the 3d and 29th of that month. Its diameter 
was estimated as before at one fourth of that of the planet."* 
Since that time, so far as we are aware, no suspicion of a satel- 
lite has been entertained by any observer. It has been urged, 
if there really be one in existence, it should have been readily 
seen at those times when Venus, like Mercury, has traversed 
the Sun's disc ; yet only two of the very many who watched 
the transits of the last century profess to have seen any object 
resembling an attendant upon the planet* Sir W. Herschel 
perceived no traces of a satellite, neither did Schroter, though 
he was most assiduous in his observations of Venus. Still it 
is not easy to undei*stand how all the observers of the last cen- 
tury can have been mistaken. In this state the question at 
present remains. 

There are several methods by which the mass of this planet 
may be ascertained. The effect of its attraction upon our 
globe causes the Sun's place to differ by a sensible quantity from 
what it should be, supposing this attraction was not in force, 
and the planet also exercises an appreciable influence on the 
precession of the equinoctial points. The most accurate inves- 
tigations show that the mass is 401,839 times less than that of 

* Professor Lambert collected the observations together, and suc- 
ceeded in deducing from them a pretty consistent orbit. The period 
of revolution assigned was lid. 5h. 13m., and the mean distance of 
the satelhte from Venus 64^ semi-diameters of the Earth, or about 
255,000 miles. The eccentricity given by Montaigne's observations 
appeared to be 0.195, and the position of the aphelion, in 1700, in 
longitude 256° : the node at the same epoch was 233°, and the plane 
of the orbit made with that of the elliptic an angle of 64°. There is 
one fatal objection to this orbit, notwithstanding its apparent agree- 
ment with observation ; if it were correct, the mass of Venus would 
be ten times greater than the value found from theory by other 
methods. Lambert's calculations will be found in Bode's Jharlmch for 
1777. 



VENUS. 39 

the Sun ; it is, therefore, a httle smaller than the mass of the 
Earth, though a nearer approach to it than obtains with any of 
the other planets. 

Transits of Venus over the Sun's disc take place under the 
same circumstances as those of Mercury, or when she has the 
same heliocentric longitude as the Earth, at or near the times 
of the nodes. The present position of the hne of nodes is in 
iongitude 75^,6 and 255^,6, and the secular sidereal motion of 
this hne is 31^, wherefore for a long time to come the transits 
of Venus must occur early in June or December, those in the 
former month at the Ascending Node, and those in the latter 
month at the Descending Node. Owing, however, to the length 
of time required after a conjunction to bring the Earth and 
planet into the same heliocentric position again, the transits of 
Venus are of rare occurrence, taking place at intervals of about 
eight and one hundred and thirteen years. They are phenom- 
ena of the highest importance as enabling astronomers to de- 
termine the distance of the Earth from the Sun, with far greater 
accuracy than any other method will give it. To do this suc- 
cessfully, it is necessary to have observations taken at places 
differing widely in latitude, so that the displacement of Venus 
upon the Sun's disc, owing to the effect of parallax, may be as 
large as possible. Now, under the circumstances that the tran- 
sits of Venus at present occur, the distance of the planet from 
the Sun is to its distance from the earth as 73 to 29, or very 
nearly as 2-^ to 1. Supposing we had observations taken at 
each of the poles of the Earth, it would be found that the dis- 
placement of Venus on the Sun's disc would occupy a space 2-J- 
times as great as the Earth's diameter, viewed from that lumi- 
nary, or five times as large as the Sun's horizontal parallax. 
Hence we see why the transits of Venus are so much more 
important than those of Mercury in the determination of this 
element, for similar considerations apphed to the latter planet 



40 THE SOLAR SYSTEM. 

will show that the displacement upon the Sun's disc, instead of 
exceeding the horizontal parallax, will be half as small again, 
so that any error entailed in the observation will have an effect 
upon the final result equivalent to more than twice the amount 
of that error. In the case of Venus, however, any error of 
observation can only influence the deduced parallax by one fifth 
of its actual amount. 

The observation consists in ascertaining the time of duration 
of a transit, or the interval elapsing between the ingress and 
egress of the planet upon the Sun's disc. The theory of Venus 
and the Sun's diameter being well-known, such observations 
readily give the parallax of the planet, and hence that of the 
Sun. 

Venus was first observed upon the Sun's disc in the year 
1639. Jeremiah Horrox, of Hoole, near Liverpool, while em- 
ployed in calculating an ephemeris of the planet from Lans- 
berg's tables, found that the geocentric latitude of the planet at 
the moment of inferior conjunction on the 24th of November, 
would be less than the Sun's semi-diamet<3r, and consequently 
that it must appear upon his disc. These tables, however, had 
so often deceived him, that he had recourse to the Tabulce 
Rudolphince., then newly published by Kepler, and based upon 
the observations of Tycho Brahe, the most exact astronomer of 
his age. According to Kepler's numbers, he found the transit 
of the planet equally certain, and, applying some corrections of 
his own, he expected to find the planet in conjunction at 4 p.m., 
on the 24th of November, about ten minutes south of the Sun's 
centre. Having thus satisfied himself that Venus must really 
appear projected upon the solar orb, he gave notice to his friend 
William Crabtree, a zealous astronomer, desiring him to observe 
it. Fortunately the planet was seen upon the Sun's disc by 
both observers, though Crabtree, interrupted by a cloudy sky, 
caught only a single glimpse of it. To make sure of the mat- 



VENUS. 41 

ter, Horrox commenced his examination of the Sun on the 23d 
of November, and repeatedly watched it nntil one o'clock, p.m., 
on the 24th, when he was called away by business. On return- 
ing at a quarter past three o'clock, he readily discerned the 
planet which had just fully immerged upon the solar disc ; in 
fact, at the first view, its outer limb coincided with that of the 
Sun. He continued his observations until a few minutes before 
sun-set. Horrox transmitted the imao-e of the Sun throuo-h a 
telescope into a darkened room, a mode of observation which 
was attended with great advantage. 

Such are the circumstances under which the planet Venus 
was for the fii-st time beheld as a black spot upon the Sun's 
disc. 

No other transit of Venus occurred until the oth of June, 
1Y61. Dr. Halley had pointed out, many years previous, that 
the parallax of the Sun could be determined within a small 
fraction of a second from observations of this phenomenon, and 
a high degree of interest was awakened as that of 1761 drew 
near. Observers proceeded from Europe to distant parts of the 
earth to secure data for ascertaining this important quantity 
with exactness, and astronomers were on the watch from Tobolsk 
in Siberia to the Cape of Good Hope. The results have been 
discussed by Professor Encke, of Berlin, in a special treatise on 
the subject ; but it is found that the individual values for the 
Sun's parallax do not agree so well as might have been antici- 
pated, and it is most fortunate that another transit, on the 3d 
of June, 1769, has aflforded more consistent numbers. 

Very extensive preparations were made for observing the 
transit of 1769. An expedition was equipped, on a laxge scale, 
and despatched to Otaheite, under the command of Captain 
Cook, and at the expense of the British Government. Continen- 
tal powers likewise joined in the preparations, and astronomers of 
various nations were sent out to the most advantageous points 



42 'J^HE SOLAR SYSTEM. 

for observation. The ingress of the planet on the Sun's disc 
was seen at almost all the observatories of Europe; the egress 
at St. Petersburg, Pekin, Orenburg, Jakutsk, Manilla, Batavia, 
&c., and the complete duration of the transit at Cape Wardhus, 
Kola and Cajeneburg in Lapland, at Otaheite, Fort Priuce of 
Wales and St. Joseph in California. The resulting parallax is 
considered certain within a very small ff action of a second of 
space ; separate investigations by Professor Encke and M. de 
Ferrer having led to precisely the same value. 

No transit of Venus has taken place since the year 1769 ; 
the next will occur on the morning of the 8th of December, 
1874, but will be invisible in this country, the conjunction hap- 
pening soon after four o'clock in the morning, and the egress of 
the planet nearly two hours before sun-rise. Another transit 
will take place on the 6th of December, 1882 ; the entrance of 
Venus on the Sun's disc will be observable in England, and her 
progress across it may be watched till sunset ; but the egress 
will not occur until eight o'clock in the evening. No transit 
will hapj^en during the twentieth century. The next, on the 
morning of the 7th of June, 2004, will be visible under favor- 
able cii'cumstances in these parts of the world.* 

Similar phenomena to those we have already noticed as at- 
tending the transits of Mercury, take place on an extended scale 
during those of Venus. A kind of lucid wave gave the first in- 

* Transits of Venus occurred as follow, according to the calcula- 
tions of M. Delambre : — 



902, November 26, 


9 A.M. 


a.d. 


, 1275, May 25, 10 p.m. 


910, November 32, 


9 P.M. 


(( 


1283, May 23, 8 p.m. 


1032, May 24, 


7 P.M. 


a 


1388, November 26, 7 a.m. 


1040, May 22, 


11 A.M. 


a 


1396, November 23, 7 p.m. 


1145, November 26, 


8 a.m. 


C( 


1518, May 26, 2 a.m. 


1153, November 23, 


8 P.M. 


IC 


1526, May 28, 6 p.m. 



The hour given being that of conjunction of the Sun and planet in 
Greenwich time. 



VENUS. 43 

timation of the planet's approach to the Sun in 1769, according 
to an observer at Greenwich ; this was followed by an apparent 
'* boihng" of the solar limb at the same place, which continued 
visible for some seconds. When the planet had partially en- 
tered upon the disc, a distortion of its outhne was remarked by 
several persons, the planet assuming an oval or elongated ap- 
pearance ; that part of it, which was still off the disc, seemed to 
be surrounded by a faint border of light. The " black drop." 
shortly before internal contact, is mentioned by numerous ob- 
servers in Europe and America, and the completion of the 
luminous thread denoting that the contact had passed was very 
generally described. Xarrow circles of light were noticed round 
the planet during its progress across the Sun, and one observer 
speaks of an illumination of the disc, possibly similar to that re- 
corded occasionally in the transits of Mercury. Similar ap- 
pearances to those noticed at the ingress have been found to 
offer themselves when the planet is leaving the Sun ; but, of 
course, in reversed order. 

The tables of Venus in present use for predicting the place 
of the planet in the heavens, are those of the Baron Lindenau, 
published at Gotha, in 1810. Within the last ten years the 
elements have been much improved by several English astrono- 
mers, with the aid of observations taken at our Eoyal Observa- 
tory. A most important addition has also been made to the 
theory of the planet by Mr. Airy, since the appearance of the 
above tables, consisting of a long inequality affecting the places 
of the earth and planet, as viewed from the Sun, to a very sen- 
sible amount. It arises from the near commensurability of the 
mean motions of the two bodies, thirteen times the period of 
Venus being nearly equal to eight times the period of the 
Earth. This inequality goes through all its changes of magni- 
tude in about 240 years, and was at a maximum about the 
commencement of the present century, when the heliocentnc 



44 THE SOLAR SYSTEM. 

place of the Earth was changed two seconds of space, and that 
of Yenus about three seconds, by this cause. At present the 
tables of Baron Lindenau are quite exact enough for all prac- 
tical purposes ; but from what has been said, it will be evident 
that astronomers have the means of improving them very con- 
siderably. 

Claudius Ptolemy has preserved for us, in his " Almagest," 
many observations of Yenus by himself and other astronomers 
before him, at Alexandria, in Egypt. The most ancient of these 
observations is dated in the four hundred and seventy-sixth year 
of Nabonassar's era, and thirteenth of the reign of Ptolemy 
Philadelphus, on the night of the lYth of the Egyptian month 
Messori, when Timocharis saw the planet echpse a star at the 
extremity of the wing of Yirgo. The date answers to b.c. 271, 
October 12 a.m. 

Similar occultations of stars and planets by Yenus have been 
witnessed in modern times. Regulus, the bright star in Leo, 
was twice echpsed by her in the sixteenth century : on the 16th 
of September, 1574, according to Msestlin, and again on the 
25tli of September, 1598, as Kepler relates in his " Astronoraise 
Pars Optica." Mars was occulted by Yenus on the 3d of Oc- 
tober, 1590, and Mercury suffered a similar eclipse on the I7th 
of May, 1737. 



CHAPTER lY. 

THE EAETH. © 

VE have now to consider the Earth on which we dwell, in its 
astronomical relations, as one of the primary planets re- 
volving round the Sun, next in order, beyond the orbit of 
Venus. 

Astronomical geography teaches us that the Earth is not a 
perfect sphere, but is somewhat flattened at the poles ; the 
equatorial diameter, therefore, being the greatest, as we shall 
presently see to be the case with the superior planets. The 
form of the Earth is, consequently, an oblate spheroid. The 
elaborate calculations of Mr. Airy, and the late Professor Bessel, 
have furnished us with a very exact determination of the actual 
dimensions of our globe. According to the former astronomer, 
the equatorial diameter measures 41,847,426 English feet, and 
the polar diameter 41, TO 7,6 20 feet. These measures, reduced 
into miles, give 7925.6 and 7899.2 respectively : the compres- 
sion at the poles therefore amounts to 26^ miles, or it is about 
l-300th part of the whole diameter. 

The transits of Venus, as already remarked, have given us 
the value of the Sun's equatorial horizontal parallax with great 
exactness. This quantity is really the angular measure of the 
Earth's equatorial semi-diameter, at our average distance from 
the Sun. Wherefore, knowing the number of miles in the di- 
ameter of our globe, we can readily ascertain by trigonometry 



46 THE SOLAR SYSTEM. 

the mean distance of the Earth from the Sun in miles, which is 
95,298,260. 

That great circle of the heavens which the Sun appears to 
us to describe in the course of a year, owing to the annual rev- 
olution of the Earth round that body, is called the ecliptic, and 
the plane of the ecliptic, or of the Earth's orbit, is employed in 
nearly all astronomical calculations as a fundamental plane of 
reference. The equator of the heavens, which is a projection 
of the terrestrial equator to the sphere of the fixed stars, makes 
an angle with the ecliptic of about 23^.27\ termed the obli- 
quity of the ecliptic. The two points where the celestial equa- 
tor is intersected by the Sun's apparent path are called the 
equinoxes ; and those where the Sun is 90^ distant from the 
equinoxes, or at his greatest north and south declinations, are 
called the solstices. The spring equinox is the point from which 
astronomers reckon the right ascensions along the equator, and 
the longitudes on the ecliptic. 

It is owing to the inclination of the ecliptic to the equator 
that, in the course of our annual revolution round the Sun, we 
experience the vicissitudes of the seasons — spring, summer, au- 
tumn, and winter. At present this inclination amounts to 
about 23*^ 27^ ; but it is subject to a very slow diminution, not 
exceeding 48'Mn 100 years. It will not always, however, be 
on the decrease, for before it can have altered 1^^, the cause 
which produces this diminution must act in a contrary direc- 
tion, and thus tend to increase the obhquity. Consequently, 
the change of obliquity is a phenomenon in which we are con- 
cerned only as astronomers, since it can never become sufficiently 
great to produce any sensible alteration of climate on the Earth's 
surface. A consideration of this remarkable astronomical fact 
cannot but remind us of the promise made to man after the 
deluge, that " while the earth remaineth, seed-time and harvest, 
mid cold and heat, and summer and winter, and day and night 



THE EARTH. 4Y 

shall not cease." The pertm-bation of obliquity, consisting 
merely of an oscillatory motion of the plane of the ecliptic, 
which will not permit of its ever becoming very great or very 
small, is an astronomical discovery in perfect unison with the 
declaration made to Noah, and explains how effectually the 
Creator had ordained the means for carrying out his promise, 
though the way it was to be accomplished remained a hidden 
secret, until the great discoveries of modern science placed it 
within human comprehension. 

Anaximander, a disciple of Thales, who was born in the 
third year of the forty-second Olympiad, or B.C. 610, is reported 
by Phny to have been the first of the ancients by whom refer- 
ence is made to the ohliquity of the ecliptic, Diogenes Laerces 
tells us that he erected a gnomon at Lacedemon, with which 
his observations were made. Other authorities attribute the 
first notice of the obliquity to Pythagoras, born about seventy 
years after Anaximander, while Diodorus Siculus, and after him 
Plutarch and Stobaeus, inform us that (Enopides of Ohio ob- 
tained the knowledge of this inclination of the equator and 
ecliptic fi'om the Egyptians. Laplace, however, makes use of 
observations for ascertaining this angle said to have been taken 
in China by Tcheou-kong 1100 years before the Christian era. 
We subjoin a table exhibiting the various determinations of the 
obliquity of the ecliptic from the earliest times to the present 
day, from which the reader will see that observation had pointed 
out its gradual diminution long before analysis was sufiiciently 
advanced to indicate the cause. Most of the aucient observa- 
tions by the Greeks and Arabians were taken with gnomons or 
armillae : their plan was to ascertain the length of the shadow 
in relation to the height of the gnomon, on the days of the sol- 
stices, when the Sun attains his greatest declinations north and 
south. Hence his altitude above the horizon could be found, 
and the difference between the results on the two solstitial 



48 THE SOLAR SYSTEM. 

epochs would give the distance between the tropics, half of which 
distance is the inclination of the ecliptic to the equator. The 
Chinese observations for taking the obhquity were taken with 
similar instruments, and are here given as reduced by Laplace in 
his paper on this subject, Connaissance des TempSy 1811 : — 

A TABLE EXHIBITING THE PRINCIPAL DETEP.MINATIONS OF THE OBLiaUITY 
OP THE ECLIPTIC, IN ANCIENT AND MODERN TIMES- 

Obliquity. 
B.C. O / " 

1100. Tcheou-kong 23 54 2 

324. Pytheas of Marseilles 23 49 20 

230. Eratosthenes of Cyrene, by observations with arniil- 

lae erected in a portico at Alexandria . . 23 51 15 

140. Hipparchus, the great astronomer . . . 23 51 15 

50. Lieou-hang 23 45 39 

A.D. 

140. Claudius Ptolemy, the Egyptian astronomer . 23 51 15 

173. Chinese observations at Layang ... 23 41 33 

461. Tsou-chong at Nan-king 23 38 52 

629. Litchun-foung at Siganfou . . . . 23 40 4 

830. Alraamun, son of the famous Harun al Raschid 23 33 52 

879. Albategnius at Aracte 23 35 

987. Aboul Wefa at Bagdad 23 35 

995. Abul Rihau with a quadrant 25 feet radius . 23 35 

1080. Arzachel in Spain 23 34 

1279. Cocheu-kong with a gnomon 40 feet height . 23 32 12 

1303. Prophatius, a Spanish Jew . . . . 23 32 

1430. Ulugh Beigh at Sarmarcand .... 23 31 48 

1460. Regiomontanus in his tables . . . . 23 30 

1587. Tycho Brahe, the celebrated Danish astronomer 23 31 30 

1660. Hevelius at Dantzic ...... 23 29 30 

1690. Flamsteed, the first astronomer at Greenwich . 23 28 56 

1750. Bradley, La Caille, &c 23 28 19 

1769. Maskclyne at Greenwich 23 28 10 

1800. Delambre and others 23 27 57 

1825. Bessel at Konigsberg 23 27 43,4 

1840. By observations at Greenwich, Edinburgh, Cam- 
bridge, and other places . . . . 23 27 36,5 



THE EARTH. 49 

The phecomenon known as \[\^ irrecession of the equinoxes 
was discovered by the celebrated astronomer Hipparchus, of 
Nicea, in the second century before the Christian era. By 
comparing his own observations of the longitudes of several 
principal fixed stars with those of Timocharis and Aristyllus, 
taken at Alexandria about 150 years previously, he found they 
differed constantly in one direction — the distances of the stai-s 
from the first point of Aries having increased apparently at the 
rate of about 1^ in a century. The effect was thus discovered, 
but the cause remained unknown till it was explained by our 
illustrious Xewton. It consists chiefly in the action of the Sun 
and Moon upon the protuberant matter at the Earth's Equator : 
a minute effect is due to the influence of the planet Venus. It 
is called the precession of the equinoxes because the equinoc- 
tial point is carried forward in reference to the circle of diurnal 
■movement, arising from the Earth's rotation on her axis ; con- 
sequently, it retrogrades upon the ecliptic, and thereby causes 
an increase in the distance of all stars from the first point of 
Anes. measiu-ed upon that circle. The present rate of progres- 
sion is about 1° 23' 4-i" in 100 years, or 50^'^ annually. Up- 
Tvards of 25,800 years will be required for a complete revolu- 
tion of the equinoctial points. 

We may conceive the phenomena of precession to arise 
from the revolution of the pole of the celestial equator, or that 
point of the heavens to which the Earth's axis is directed, round 
the pole of the ecliptic, in the period of 25,800 years, at a mu- 
tual inchnation of 23^ 28', and shall thus obtain an insight into 
the nature of another important inequahty, called the nutation 
of the Earth's axis, which exercises very appreciable influence 
upon the positions of the stars, as we see them from the Earth, 
and goes through all its variations in somewhat less than nine- 
teen years, or in the course of one revolution of the Moon's nodes. 
The same cause which operates in producing the precession of 

3 



50 THE SOLAR SYSTEM, 

the equinoxes gives rise also to the nutation, in virtue of which 
the Earth's axis, instead of being continually directed to the 
same point in the celestial sphere, describes a small ellipse on 
the surface of the heavens, having the ratio of the greater axis 
to the lesser as 3V to 27, the length of the major axis in arc of 
a great circle being 18 ^'.6. 

Dr. Bradley fii-st discovered and explained the nutation of 
the Earth's axis, soon after he had been led by his own obser- 
vations to infer the existence of the aberration of light. We 
have, therefore, to thank this eminent astronomer for two of 
the most important discoveries connected with the science. 

The Earth's orbit is not circular, but an ellipse of very mod- 
erate eccentricity, the perihelion point in 1800 answering to 
279^ 30' 8^' of heliocentric longitude. The line of apsides is 
subject to an annual direct change of 11.29", independent of 
the effect of precession, so that, allowing for the latter cause of 
disturbance, the annual movement of the apsides in reference to 
the variable equinox may be roughly taken at one minute of 
arc. One important consequence of this slow motion of the 
gi'eater axis of the Earth's orbit is the variation in the length 
of the seasons in different centuries. In the time of Hippar- 
chus, the longitude of the Sun's perigee was between the au- 
tumnal equinoctial point and the winter solstice, and the au- 
tumn w^as the shortest of the seasons ; spring was longer than 
summer, and winter longer than autumn. About the middle 
of the thirteenth century the perigee coincided with the winter 
solstice, whence spring was equal to summer, and autumn to 
"winter. In the year 1850 we find the time elapsing between 

d. h, m. 
The spring equinox and summer solstice . 92 20 67 
The summer solstice and autumnal equinox . 93 14 
The autumnal equinox and winter solstice . 89 17 38 
The winter solstice and spring equinox , . 89 1 17 



THE EARTH. 51 

Hence the spring has become shorter than the summer, and the 
autumn longer than the winter. 

About four thousand years before the Christian era, or 
singularly enough, near the epoch of the creation of man, ac- 
cording to chronologists, the perigee coincided with the vernal 
equinox., and the winter and spring were equal, and shorter 
than the summer and autumn, which were also equal. In 
A.D. 6485, or thereabouts, the perigee will have completed half 
a revolution, and will then coincide with the autumnal equi- 
nox ; summer will be equal to autumn, and winter to spring, 
but the former seasons will be the shortest. All these changes, 
it is to be observed, are due in the first place to the eccen- 
tricity of the Earth's orbit, and to the progressive movement 
of the line of apsides. The eccentricity, as we have stated 
above, is not large ; at the commencement of the present cen- 
tury it amounted to 0.0167923, but is subject to a slow dimi- 
nution, not exceeding 0?000044 in one hundred years. Sup- 
posing this change permanent, the Earth's orbit must eventually 
become circular, but the theory of attraction enables us to 
prove that the diminution is not to continue beyond a certain 
time, and although we are not yet in a condition to assign def- 
inite hmits to the oscillations, we know that after the lapse of 
many thousand years the eccentricity will be stationary for a 
time, and afterward increase; and, without some external 
cause of perturbation, these variations, within certain not very 
distant limits, must continue throughout all ages. 

Were the Earth's orbit circular, and the plane of the equa- 
tor coincident with that of the ecliptic, the Sun would appear 
to describe an equal arc of the heavens day after day, and con- 
sequently the interval elapsing between two successive passages 
over the meridian of any place on the Earth's surface would 
be sensibly the same, and the solar day would be something 
like an equable period of time. But, as we have seen, the 



52 THE SOLAR SYSTEM. 

Earth's path round the Sun is elliptical, and the apparent diur- 
nal velocity of the Sun varies with our distance from him. 
Moreover, the equator is inclined to the plane of her path at 
an angle of about 28^ 28', and this again has a great influence 
on the length of the arcs of Right Ascension^ passed over by 
the Sun on successive days. It follows, therefore, that the 
solar day, or the interval elapsing between two consecutive 
meridian transits of the Sun, is of variable length. To secure 
an equable measure of time, astronomers assume the revolu- 
tion of a mean Sun in the plane of the equator with the real 
Sun's mean diurnal motion in Right Ascension, and the time 
intervening between two successive transits of the mean Sun is 
called a mean solar day, which is the unit of time in common 
use at present. 

The difference between the Right Ascension of the mean 
and true Suns is termed the Equation of Time^ and in order 
to regulate a clock by observations of the time of culmination 
of the Sun it is necessary to know the amount of this equation 
for each day, and we, accordingly, have it tabulated in astronom- 
ical ephemerides and almanacs. The equation is at a maxi- 
mum about the 10th of February, when an additional correc- 
tion of about 14m. 32s. is required to reduce apparent solar 
time to mean solar time, or, in other words, the mean Sun is 
on the meridian 14m. 32s. before the true Sun. On the 15th 
of April there is no equation, the real and imaginary Suns 
beino; on the meridian at the same moment. In the middle 
of May the equation again reaches a maximum at 3m. 5 4s., 
and disappears on the loth of June. Another maximum 
occurs about the 27th of July, when a correction of 6m. lis, 
is required to be added to apparent solar time. On the 1st of 
September it again vanishes, but increases from that time until 
the beginning of November, when the equation amounts to 
16m. lYs., subtractive from apparent time, and again becomes 



THE EARTH. 53 

zero about the 25tli of December; thus there are four maxima 
in each year. 

The sidereal day is the time intervening between two con- 
secutive passages of the same star over a meridian ; it is con- 
sequently the length of the Earth's diurnal rotation upon her 
axis, and expressed in mean solar time is 23h. 56m. 4.9s. The 
sidereal day is subject to no sensible variation. It is in conse- 
quence of the acceleration of the sidereal upon the mean solar 
day that the aspect of the heavens is varied at different times 
of the year, those stars which at one time appeared on the 
meridian at midnight gradually gaining upon it, until they are 
lost in the western heavens at sunset, to make their reappear- 
ance in the east after the lapse of a few months. 

The interval of time in w^hich the Sun appears to us to 
complete an entire circuit of the heavens, in reference to the 
fixed stars, is called by astronomers a sidereal year^ and con- 
sists of 365d. 6b. 9m. 10.7s. In this period, however, the 
equinox will have retrograded 50^'^, and the Sun will reach 
the same point of longitude from which he started in a shorter 
time, than would be required to elapse from the moment of his 
leaving a fixed star until he again returns to it. The revolu- 
tion in respect to the equinox is thus shorter than the revolution 
in respect to the stars, by the interval occupied by the Earth 
.in passing over an arc of bO\" upon her orbit, or by Oh. 20m. 
22.9s., and we obtain 365d. oh. 48m. 47. 8s., for the length of 
the revolution in reference to the equinoctial point, or, as it is 
termed, the trojncal year. 

We have seen that the perihehon point of the Earth's orbit 
has an annual motion amongst the stars of about eleven sec- 
onds, by which quantity the longitude exceeds that of the pre- 
ceding year. If the Sun start from the place of the perihelion 
he will require a longer interval of time than the sidereal year 
to reach it again, and the excess will be equal to the time 



54 THE SOLAR SYSTEM. 

necessary for the Earth to describe 11.29s. of her orbit, or 
4m. 35.0s., which gives us 365d. 6h. 13m. 45. Vs. for the dura- 
tion of what is called the anoinalistlc year. This period is 
occasionally used in astronomical investigations, but mankind 
generally are more concerned in the tropical yeai\ on which 
the return of the season depends. This year is subject at pres- 
ent to a slow diminution, amounting to little more than half a 
second in one hundred years, yet, like all variations of the 
kind by which the Earth's orbit is affected, the diminution is 
not to continue forever. 

The ancient Egyptian year consisted of 365 days, as we 
learn from Herodotus. The Thebans, or inhabitants of Upper 
Egypt, are said to have perceived the necessity of an addition 
of six hours to this period, in order to make it agree with the 
annual course of the Sun. In the time of Democritus, about 
450 yeai-s before Christ, the year was supposed to consist of 
365^- days. Eudoxus, who flourished soon afterwards, con- 
sidered it somewhat longer, while CEnopides, of Chius, men- 
tioned by Diodorus, made it 365d. 8h. 48m. The Cahppic 
period of 76 years, commencing at the death of Darius, 
B.C. 329, consisted of 27,759 days, giving 365^- days for the 
length of the year. 

The great astronomer, Hipparchus, found by his own ob- 
servations that the year of Calippus was somewhat too long, 
and, accordingly, diminished it by the three-hundredth part of 
a day, or 4m. 48s., whence he fixed the length of the tropical 
year at 36od. 5h. 55m. 12s., differing little more than six 
minutes from the best modern determination, i^early three 
hundred yeai's after the time of Hipparchus, Ptolemy appeai*s 
to have investigated the length of a year, but concluded by 
adopting the duration assigned by the Greek astronomer, and 
employs it in the Solar Tables found in his Almagest. 

The Arabian Prince Albategnius, who lived at the latter 



THE EARTH. 



55 



end of the ninth century, and observed at Aracte, in Chaldea, 
perceived the want of some correction to the Ptolemaic or 
Hipparchian year, and by comparison of his own observations 
with those at Alexandria, inferred that the length of the tropi- 
cal year Avas 365d. oh. 46 ra. 24s., as reported in his work De 
Scientia Stellar um. In the Alphonsine Tables, compiled 
about 1252, by Alphonsus X., King of Castile, with the as- 
sistance of the best astronomers of his age, we find the length 
of the year 36ocl. 5h. 49m. 16s., a very near approximation to 
the truth. 

Since this epoch, the duration of the tropical year has 
formed the frequent subject of investigation. The following 
table exhibits the principal determinations up to the present 
time : — 

d. h. m. s. 

Nicolas Copernicus, in 1543 365 5 49 6 

Tycho Brahe, in 1602 365 5 48 45i 

Kepler, in the Tabulm Riidolphince . . . 365 5 48 57.6 
Cassini, in 1743, by comparison of his own 

observations of Equinoxes with those 

of previous observers 365 5 48 52.4 

Flamst^ed, our first Astronomer Royal . 365 5 48 57.5 

Halley, in his Astronomical Tables . . 365 5 48 54.8 

Lacaille, in his Tables 365 5 48 49 

Bessel, in 1830, gives for 1800 .... 365 5 48 47.8 



CHAPTEE V. 

THE MOOI^. (B 

THE Moon, the constant attendant of the Earth in her an- 
nual course round the Sun, is bj far the nearest to us of all 
the heavenly bodies, being situated at an average mean dis- 
tance of only 238,650 miles. To her we are indebted, not 
merely for illumining by her presence our dark winter nights, 
but hkewise for that more important phenomenon, the tides of 
the ocean, in the production of which she has the greatest in- 
fluence, and it is not unlikely that this extraordinary luminary 
exercises an effect upon the Earth in other ways, of which we 
are not at present fully cognizant. 

The interval of time occupied by the Moon in performing 
one sidereal revolution round the earth,* or the time w^hich 
elapses between her leaving a fixed star until she again returns 
to it, is found by the latest and most accurate investigation, to 
be 2'7d. 7h. 43m. 11.461s., whence we find the mean tropical 

* Strictly speaking the centre of the Earth is not the point round 
which the Moon revolves ; but both bodies have a revolution round 
their common centre of gravity. This point is situated at an average 
distance of 2,690 miles from the centre of our globe, or about 1.270 
miles beneath the surface, and subject to a variation of rather more 
than 165 miles, one way or the other, in consequence of the variable 
distance between the Earth and the Moon. The perturbation in the 
place of the Earth, or rather its reaction on the place of the Sun, 
owing to this motion round the centre of gravity, may affect the Sun's 
longitude more than five seconds of arc, and his latitude about 0.7''. 



THE MOON. 57 

revolution 2'7d. '7h. 43m. 4.614s., since the equinox will have 
receded in a sidereal period 3* 7 5 8^', a space which the Moon 
would require 6*847^^ to traverse with her average mean 
motion. 

The phenomena, termed the ^;Aa5e5 of the Moon, do not 
recur in the space of a sidereal revolution, for it is evident that 
the real motion of the Earth in that interval, giving rise to an 
apparent motion of the Sun, in the direction in which the Moon 
revolves, must cause the period between two conjunctions or 
oppositions to be longer than either the sidereal or tropical rev- 
olutions, the Moon having to traverse an arc equal to the angu- 
lar movement of the Earth in the sidereal period, before she is 
again in a line (to speak roughly) with the Earth and Sun. 
The lunar month, or as it is usually called by astronomers, the 
synodical revolution of the Moon, is consequently longer than 
the sidereal period, and exceeds it by 2d. 5h. Om. 51*41s., 
which is the time required by that body to describe, with her 
mean angular velocity of 13" 1764° per day, the arc traversed 
by the Sun since the previous conjunction. Hence we find the 
duration of the synodical period to be 29d. 12h. 44m. 2'87s. 

The lunar phases depend on the position of the Moon with 
respect to the Sun, or what amounts to the same thing, her dis- 
tance from conjunction, which is termed in astronomical lan- 
guage, the age of the Moon. Being an opaque spherical globe, 
reflecting the Sun's light, she can only appear fully illuminated 
when opposite that luminary, and in all other positions her illu- 
minated disc appears less than a circle. Soon after conjunction 
with the Sun, she maybe seen as a very narrow crescent, a little 
above the western horizon at sun-set, for being then between 
the Earth and Sun, her illuminated surface is in a great measure 
turned fi'om us. As she advances in her orbit, the dark part 
gradually diminishes until the Moon is 90"^ from conjunction, 
which is called the first quarter, and then the bright and unil- 

3* 



58 THE SOLAR SYSTEM. 

luminated parts are equal. After this point, the illuminated 
surface increases till the Moon is in opposition, when it is said 
to he full,, and presents to us her whole enlightened disc. The 
bright part then begins to diminish and again occupies one 
half of her surface when the Moon is 90^ from the conjunction, 
at the last quarter, becoming narrower as she approaches it, till 
a thin crescent above the eastern horizon shortly before sun-rise 
is all that rtoains. These phases are consequently recurrent 
after the interval of a synodkol revolution, and depend upon 
the position of the visible^ in reference to the enlightened^ hemi- 
sphere. 

The eccentricity of the lunar orbit is considerable, and from 
this cause, the distance between the Earth and the Moon, at the 
perigee, may be no more than 225,560 miles, while at apogee 
it may increase to 251,700 miles, the ellipticity of the orbit 
therefore producing a variation in the length of the radius vec- 
tor, or true distance. from the Earth, of rather more than 26,000 
miles. The mean inclination of the orbit to the ecliptic is, ac- 
cording to recent determination, 5° 8^ 55*46^', but this is sub- 
ject to a variation, one way and the other, of rather less than 
23': the latest tables of the Moon giving the greatest iuchna- 
tion 5° 20' Q", and the least, 4^ 57' 22''. The line of nodes 
of the lunar orbit revolves round the ecliptic in 18yrs. 218d. 
21h. 22m. 46s., in a retrograde direction, which is at the rate 
of 1^-° in each sidereal period, or somewhat more than 3' daily. 
This retrogression of the nodes is caused by the action of the 
Sun, w^hich modifies the central gravity of the Moon towards 
the Earth. It is not, how^ever, an equable motion throughout 
the whole of the Moon's revolution ; the node, generally speak- 
ing, is stationary when she is in quadrature, or in the ecliptic ; 
in all other parts of the orbit it has a retrograde motion, which 
is greater the nearer the Moon is to the syzigies, or the greater 
the distance from the ecliptic. The preponderating effect at the 



TRE MOON. 59 

end of each synodic period is, however, retrocessive, and gives 
rise to the revolution of the Hne of nodes in between eio:hteen 
and nineteen yeai-s. Hence it is necessary to distinguish be- 
tween the mean place of the node which supposes a regular 
movement of 3' 10'^ daily, contrary to the order of signs and 
its true place at any time. The backward movement of the 
line of nodes was discovered by the ancients, but first explained 
by Newton. 

The line of apsides or major axis of the lunar orbit has, from 
a similar cause, a direct motion on the ecliptic and accomplishes 
a whole revolution in 8yrs. 310d. ISli, 48m. 5 3s., so that in 
4yrs. lood. the perigee arrives where the apogee was before. 
This motion of the line of apsides, like the movement of the 
nodes, is not regular and equable throughout the whole of a 
lunar month, for when the Moon is in syzigies the hne of apsides 
advances in the order of signs, but is retrograde in quadratures. 
But the preponderating effect in several revolutions tends to 
advance the apsides, and hence arises their direct revolution in 
between eight and nine yeai-s. 

The apparent diameter of the Moon, at her mean distance 
from the Earth, is 31' 19-8'', but the eccentricity of her orbit 
causes a variation of about 4' 44 '\ the maximum beincr 33' 
32'', and the minimum 28^ 48'^ According to Professor 
Madler, the real diameter of the Moon is 2159*6 English miles; 
the recent measures of Dr. Wichmann make it 2162 miles, 
agreeing so nearly with the former value, that we may conclude 
we know the dimensions of the lunar orb very exactly. Takino^ 
the diameter at 2160 miles, which cannot be far from the truth, 
the circumference will be 6786 miles, and the bulk of the Moon 
will be to that of the Earth as 1 to 49-J-. Dr. Wichmann could 
not detect a sensible difference between the equatoiial and polar 
diametei*s. 

Some of the best determinations of the mass of the Moon 



60 THE SOLAR SYSTEM. 

depend upon the amount of lunar nutation, which, as we have 
seen, is a periodical fluctuation in the position of the Earth's 
axis, arisincr from the variable direction of the line of nodes of 
the Moon's orbit. The Baron Lindenau, from his researches on 
nutation, concluded the mass to be l-S^'Y of that of the Earth ; 
Professor Henderson has more recently given l-YS'Q. There 
are various other methods of ascertaining the mass of our satel- 
lite, as, for instance, the observation of the change in the Sun's 
longitude due to her attraction on the Earth. Mr. Airy's 
method of observing the positions of Venus near her inferior 
conjunction, already mentioned, which is similar in principle to 
the last, and the investigations of her effect upon the tides.* 
Roughly speaking, we shall be near the truth if we assume the 
mass of the Moon to be 1-8 0th of that of the Earth. 

The most casual observer of the Moon can hardly fail to 
have remarked, that she always presents very nearly the same 
face towards us, and a Httle reflection will convince him that 
the cause must he in the near equality of her periods of axial 
rotation and sidereal revolution round the Earth. Were these 
periods exactly equal, we should have the same hemisphere 
turned towards us without the slightest variation ; but the or- 
bital period is subject to small irregularities, while the time of 
axial rotation remains constant, and for this reason a phenome- 
non termed the lihration takes place, whereby we occasionally 
see a little more of one edge of the Moon than usual, either on 
the eastern or western sides of her equatorial regions. Galileo 
was the first astronomer who remarked this periodical variation 
in the visible surface of the Moon, and the circumstance reflects 
no little credit on his zeal and attention, for his instrumental 
means are well known to have been very small, notwithstanding 
the numerous discoveries we owe to him. Generally speaking, 

* By this method Laplace coucluded the mass of the Moon to be 
l-73d of that of the Earth. 



THE MOON. 61 

the Moon revolves on her axis in the period of one mean side- 
real revolution. 

The axis of the Moon is not quite, though very nearly, per- 
pendicular to the plane of her orbit, which allows of our seeing 
a little more of the polar regions at certain times than at others, 
a phenomenon called the Uhration in latitude. The angle be- 
tween the lunar equator and the plane of her path round the 
Earth is, according to Nicollet, 1^ 28^ 47^', or, according to the 
more rect-it and elaborate researches of Dr. Wichmann, 1° 32' 
9". 

By WiQ parallactic Uhration we understand the difference in 
the position of a spot as viewed from the centre and surface of 
the Earth. 

Professor Bessel and Dr. Wichmann have made some re- 
searches respecting the existence of a physical Uhration^ or a 
real variation in the time of axial rotation of the Moon, such as 
would give rise to an apparent change in the position of the 
spots, periodical or otherwise ; but there does not at present 
appear to be sufficient reason for suspecting any inequality of 
this nature. 

The full Moon which falls nearest to the Autumnal Equinox 
has long received the name of the Harvest Moon^ from the fact 
that the difference between the hours of her rising on two suc- 
cessive evenings is then at a minimum, and the long duration 
of moonhght, thus afforded soon after sun-set, is most advanta- 
geous to the farmer at this critical season. This near coinci- 
dence in the times of several successive risings takes place every 
lunar month, when the Moon is in the signs Pisces and Aries, 
but it has only been remarked when she is at full in these signs, 
and this can only happen in August or September. The least 
possible difference between two successive risings in this latitude 
is about seventeen minutes. When the Moon is in Libra, and 
at the same time near the descending node of her orbit upon 



02 THE SOLAR SYSTEM. 

the ecliptic, the difference between the times of rising on two 
evenings is the greatest possible, and in London will amount to 
about Ih. 16m. 

The theory of the lunar motions is, perhaps, the most diffi- 
cult with which the astronomer has to deal, and it has accord- 
ingly occupied the attention of the most eminent mathema- 
ticians, from the time of Sir Isaac Newton to the present day. 
To bring it to the exact and elaborate form in which it now is, 
has required the utmost efforts of the observer as well as the 
physical astronomer, for it is a curious fact, that some of the 
most important of the smaller inequalities, as they are termed, 
of the Moon's mean motion, have been first detected by actual 
observation, and subsequently reconciled with the theory of 
gravitation as expounded by Newton. The larger inequahties 
are of such magnitude, that they were discovered with the rude 
instruments employed by Hipparchus in the second century be- 
fore the Christian era. It would lead us beyond the limits and 
plan of the present work, were we to enter into any explanatory 
account of the lunar perturbations generally, which, after all 
that can be said respecting them, are not easily intelligible 
without a knowledge of the mathematical processes by which 
they have been detected and reduced to calculation. We shall, 
therefore, content ourselves with a brief notice of the most im- 
portant inequahties affecting the mean longitude of our satellite. 

The most considerable is that termed the Evect'ion^ the dis- 
covery of which we owe to the famous astronomer Hipparchus, 
in the second century before the Christian era. It depends 
upon the angular distance of the Moon from the Sun, and the 
mean anomaly of the former, and goes through its variations 
in a period of about 3 Id. 19h. 30m. At its maximum it may 
influence the Moon's longitude one way or the other about 1° 
20'. It diminishes the equation of the centre in syzigies, and 
increases it in quadratures. 



THE MOON. 63 

Another large inequality is called the Variation^ the dis* 
covery of which has usually been attributed to Tycho Brahe, 
though M. Sedillot and others have claimed the merit of its 
first recognition for the Arabian astronomer, x\boul Wefa, who 
lived in the ninth century. It depends solely on the angular 
distance of the Moon from the Sun, and its period is half a 
synodic revolution, or about 14d. 18h. The effect of this ine- 
quahty is greatest in the octants, and disappears in syzigies and 
quadratm-es, the longitude of the Moon being altered thereby 
rather more than half a degree when the equation is at a max- 
imum. The Variation was the first lunar inequality explained 
by Sir Isaac Newton upon the theory of gravitation. 

The parallactic inequality arises from the sensible difference 
in the Sun's disturbing force, when the Moon is travelling over 
that semi-circumference of her orbit lying near the Sun, and 
when she is in the further semicircle. Small as is the change 
of distance at these times, the perturbation depending upon it 
is sufficiently large to produce an inequality, which at its max- 
imum may alter the Moon's longitude about two minutes of 
arc. The period during which it passes through all its varia- 
tions is one synodical revolution, or 29d. 12h. 44m. 

The Annual Equation is an inequality caused by a varia- 
tion in the angular motion of the Moon, w^hich becomes slower 
as the Earth and Moon are approaching the Sun, and acceler- 
ates as they recede from him. The motion of our satellite is 
slower than the mean motion during the time the Earth is 
moving from perihelion to aphelion, and more rapid as she 
passes from aphelion to perihelion, or in the present position of 
the line of apsides, the Moon moves slower between the end 
of December and June, than between the end of June and the 
end of December. The period of this inequality is the anom- 
aUstic solar year, and its maximum effect upon the Moon's lon- 
gitude amounts to 11' 10". 



64 THE SOLAR SYSTEM. 

The Secular Acceleration of the Mood's mean motion is 
caused by a change in the eccentricity of the Earth's orbit, 
which has been slowly diminishing for many ages. It was dis- 
covered by Dr. Halley from a comparison of the periodic time 
of the Moon, deduced from recent observations, with that indi- 
cated by the Chaldean observations of eclipses at Babylon in 
the years 719 and 720 before the Christian era, and the Ara- 
bian observations in the eighth and ninth centuries, the result 
of which showed that the periodic time is now sensibly shorter 
than at the epoch of Chaldean eclipses. The cause of this 
diminution was not understood until the celebrated Laplace 
showed that it was similar in its origin to the much larger and 
more raj^id fluctuation of period, which takes place according 
to the position of the Sun with respect to the line of apsides 
of the lunar orbit. The mean motion of the Moon is increased 
by this inequality, at the rate of rather more than ten seconds 
in one hundred years. As the diminution of eccentricity of 
the Earth's orbit is not a permanent change, though extending 
over a period whose duration is hardly yet calculable, so the 
acceleration of the Moon's mean motion is a cyclical inequality, 
and a time will arrive when it must altogether cease. After 
this remarkable equation had been detected by Dr. Halley, 
great doubts existed in some minds as to the possibiUty of ex- 
plaining it on the theory of gravitation. The elucidation which 
the subject has received at the hands of Laplace is, therefore, 
the more remarkable, and affords one of many instances where 
suspicions of the failure of Xew ton's law have ultimately tended 
only to its more striking confirmation. 

The Tables at present in general use for predicting the 
positions, echpses, and other phenomena of our satellite, are 
those calculated by the French astronomer Burckhardt, and 
published at Paris in 1812. They are founded principally upon 
the observations taken at our National Observatory at Green- 



THE MOON. 65 

wich, an establishment which was instituted with an especial 
view to the improvement of the lunar theory, and thereby, of 
the art of navigation. Burckhardt's tables are used in the 
preparation of the Xautical Ephemerides annually issued by 
the governments of Great Britain, France, and Prussia. The 
late Baron Damoiseau was the author of two sets of lunar 
tables, wbich are based upon elements not very different from 
those employed by Burckhardt. The first set, according to the 
centesimal division of the circle, appeared in 1824, and the 
second, agreeably to the old or sexagesimal division, in 1828. 
Since the investigation of the elements of the existing tables, 
the lunar theory has been greatly improved by the laborious 
researches of M. Plana of Turin and Professor Hansen of See- 
berg, and several inequahties of long period have been disco\"- 
ered, which almost entirely reconcile the differences between 
the observed and tabular places of the Moon. But the most 
important w^ork undertaken for perfecting our knowledge of her 
movements, and one which is hardly equalled for magnitude 
and intricacy in the history of astronomy, is the reduction of 
all the observations of the Moon taken at the Royal Observa- 
tory, Greenwich, between the years 1750 and 1830, which has 
been brought to a completion within the last few years, under 
the superintendence of the Astronomer E-oyal. There are about 
8000 observations in all, and, for the attainment of the object 
in view, it was necessary to reduce the whole again, with the 
best modern elements, to compute the tabular places in dupli- 
cate, the tables themselves having been modified and extended 
so as to accord as nearly as possible with M. Plana's theory, and 
finally to determine the pricipal elements of the Moon's m.otion, 
from the whole mass of observations. Few persons who have 
not had an opportunity of viewing the manuscripts themselves 
can form any adequate idea of the enormous labor attending 
this valuable work. Twelve computers on an average were 



QQ THE SOLAR SYSTEM. 

engaged eight hours a-day for several years, the reductions 
having been commenced in earnest in August 1841, and the 
last sheets of the second large volume containing the results 
having passed throuo-h the press in the spring of 1848. Pro- 
fessor Hansen has undertaken a complete revision of the lunar 
theory, having at his command, where necessary, the Green- 
wich reductions of the 8000 observations, and the British Gov- 
ernment has lately provided funds to aid this distinguished 
mathematician in his important inquiry."^ The Astronomer 
Royal communicated to the Royal Astronomical Society several 
years since, the most prominent conclusions at which he had 
himself arrived after discussing the Greenwich observations. 

The naked eye readily discerns that the disc of the full Moon 
is not uniformly bright : hght and dark regions alternate upon 
it, giving the idea of continents and seas analogous to those on 
our own globe. In fact, the earlier selenographists considered 
the dull grayish spots to be water, and termed them the lunar 
seas, bays, and lakes. They are so called to the present day, 
though we have strong evidence to show that if water exist at 
all on the Moon, it must be in very small quantity. On apply- 
ing the telescope, with suitable magnifying powei-s, we perceive 
on every part of the surface, even in the midst of the so-called 
oceans and seas, annular spots evidently of a volcanic charac- 
ter, with extensive chains of mountains and steep isolated rocks, 
forming altogether a very rugged and desolate appearance. The 

* As one of the early fruits of this investigation, we may mention 
the discovery of two inequalities in the motion of our satellite, result- 
ing from the attraction of Venus, exercised directly in one instance 
and indirectly in the other. Great importance is attached to these 
discoveries, because, when their influence is taken into account, the 
positions of the Moon, calculated from theory, are almost precisely 
identical with those given by observations, which renders it certain 
that our knowledge of the movements of our nearest celestial neigh- 
bor is very nearly perfect. 



THE MOON. eY 

craters are exceedingly numerous ; in some places they are 
thickly crowded together, small volcanoes having formed on 
the sides of the larger ones, — in other regions they are com- 
paratively isolated. Their dimensions are far greater than 
those of the largest volcanoes on the Earth, the breadth of the 
chasm occasionally exceeding one hundred miles, while the 
sides of the mountain attain a very considerable elevation. The 
best time for viewing a crater is when it is just clear of the dark 
part of the Moon, or when the Sun is just above its horizon : 
we can then trace the shadows thrown by the sides of the 
mountain upon its interior and exterior surface, and, by meas- 
uring the lengths of these shadows, we may approximate to 
its true altitude. Some of the steep isolated rocks throw their 
shadows for many miles across the plains surrounding them. 

The positions of the lunar spots upon the surface are usually 
given in selenographic longitudes and latitudes. In the large 
work of Professor Madler are found the results of a great num- 
ber of observations for fixing the exact places of the principal 
mountains and craters, first in longitude and latitude, and after- 
wards in the form of co-ordinates, to facilitate the construction 
of the lunar chart. The first quadrant contains west longitude 
and north latitude ; the second, east longitude and north lati- 
tude ; the third, east longitude and south latitude ; and the 
fourth, west longitude and south latitude. 

To distinguish the various spots from eSch other, two 
nomenclatures have been adopted by Hevelius and Riccioli re- 
spectively. The former made use chiefly of the names of places 
upon the Earth, the latter introduced the names of celebrities 
of all ages in science and literature ; and this method is the 
one adopted by Professor Madler, the greatest selenographer of 
the present day, and, iu fact, universally followed by astrono- 
mers. Amongst the English names thus appropriated ai-e 
those of Newton, Flamsteed, Bradley, Maskelyne, Airy, Dol- 



68 THE SOLAR SYSTEM. 

lond, Cook, Herschel, Ramsden, Sabine, Wollaston, &c., and 
the names of nearly all the foreign astronomers and mathema- 
ticians of eminence in past and present times are found on 
Madler's large chart of the Moon. This nomenclature is cer- 
tainly open to more than one objection. " The neutral ground 
of mythology and classic antiquity," to use Sir John Herschel's 
expression, would perhaps have been the safest and most lasting- 
foundation. It is true many mythological names occur already 
on the lunar maps ; but it is perhaps to be regretted that they 
have not been more extensively employed, to the exclusion of 
modern names altogether. When another rigorous telescopic 
survey of the Moon's disc is undertaken, as it probably will be 
some years hence, we think it is unlikely that an alteration will 
be found necessary. 

One of the most remarkable of the lunar spots is that 
called Tycho, which is readily distinguished in the southern 
part of the Full Moon by the number of luminous rays or 
streaks of light which diverge from it, particularly in a north- 
easterly direction. Tycho is the Mons Sinai of Hevelius, and 
lies in 12^ east longitude and 43^ south latitude: it is an an- 
nular mountain or crater of not less than fifty-four English 
miles diameter. The height of the western border above the 
interior level is, according to Madler, 17,100 feet, and of the 
eastern border rather more than 16,000 feet. A mountain, 
very nearly one mile high, marks tbe centre of the crater. 
Tycho is surrounded on all sides by a great number of craters, 
peaks, and ridges of mountains, lying so close together that in 
some directions it is impossible to find the smallest level place. 
It is, as above remarked, the origin of a number of luminous 
streaks or rays, which extend therefrom over fully the fourth 
part of the Moon's disc; the brightest ones branch ofiF in a 
north-easterly direction, and there are others yerj conspicuous 
on the western side of the crater. These rays become visible 



THE MOON. 69 

as soon as the Sun is elevated from 20° to 25° above their ho- 
rizon. The color is, perhaps, a little whiter or more silvery 
than the general lunar surface. Many opinions respecting the 
nature of these appearances have been advanced by Cassini, 
Schroter, Herschel, and others, and they have been variously 
styled mountains, streams of lava, or even roads : there is 
nothing on the Earth's surface bearing the slightest analogy to 
them. Perhaps the idea first started by Mr. Nasmyth is the 
most probable, that they have been caused by a general volcanic 
upheaving of the crust of the Moon in former times, which has 
produced an appearance on the lunar surface similar to that of 
a pane of glass broken by a sharp-pointed instrument. The 
mere fact of their diverging fiom the great crater Tjcho, proves 
that it was the focus of the volcanic outbreak, whenever it may 
have occurred. A sharp eye will easily detect the appearances 
we have described on the full Moon, without the assistance of 
a telescope. 

Another very beautiful annular mountain, the converging 
point of similar luminous streaks, is that known as Copernicus^ 
whose selenographic place is in longitude 20° E. and latitude 
9° N. The diameter of the crater is somewhat larger than in 
the former case, or rather more than fifty-five miles. The 
highest point is about 11,250 feet above the surrounding plain. 
It is readily discernible on the full Moon, but is most favorably 
viewed when the Sun's rays have just reached its eastern side 
about the time of quadrature or first quarter, the shadows of 
the western side of the crater being then thrown on the interior 
level, that of the central peak on the same level towards the 
eastern side, while the shadow of this side of the mountain 
darkens for some distance the exterior plain on the rugged edge 
of the Moon. Generally speaking, these shadows are extremely 
well defined and very black. , 

The spot itself is most advantageously seen in these lati- 



'70 THE SOLAR SYSTEM. 

tildes about the first quarters in the spring, when the Moon has 
a high northern dechnation, but the divergent streams of light 
will be best observed near the time of full Moon. They vary 
in breadth from three to ten miles ; the principal ones branch- 
ing off from the crater towards the north-east. 

Kepler is a conspicuous annular mountain, the focus of 
similar rays of light, in longitude 38^ E. and latitude 8^ N. 
The crater itself is about twenty-two miles in diameter, and the 
altitude of its eastern edge above the level of the interior is 
found to be more than 10,000 feet. This mountain is situated 
on the Oceanus Procellarum, the largest of the lunar seas, 

Tycho, Copernicus, and Kepler, are the principal mountains 
of the class termed by Professor Madler '' Strahlenbergen," or 
the craters which form the radiating point of the streaks or 
rays, which appear so remarkable on the surface of the full 
Moon. 

Eratosthenes is a very beautiful annular mountain placed at 
the extremity of the long range called the Apennines, which 
cover a surface of more than 16,000 square miles. It is the 
Insula Yulcania of Hevelius, " the mighty key -stone of the 
Apennines," as it is aptly termed by Madler. The crater is not 
less than thirty-seven miles in diameter ; its centre is occupied 
by a steep rock 15,800 feet in altitude above the level surface 
of the interior. The outside of the circular mountain is about 
3300 feet above the plain, on the western border, while on the 
eastern side the height is more than twice as great. Eratosthe- 
nes is a beautiful spot about the time of first quarter, when the 
long range of mountains running off from it in a semicircular 
form, and their shadows thrown upon the broad plain west of the 
crater, are most interesting objects in a telescope of adequate 
power. The selenographic place is in longitude 11° 4' E. and 
latitude 14*^ 4' N. It is, consequently, situated to the IST. W. 
of Copernicus already described. The highest of the Apennine 



THE MOON. 



1\ 



range is a mountain called Huyghens^ which is somewhat more 
than 18,000 feet in altitude. Bradley is another lofty peak of 
the same range, and, according to Madler, attains an elevation 
of 13,500 feet. 

Manilius is a beautiful annular mountain on the north side 
of the Mare Vaporum, and is the best determined point on the 
lunar surface. M.M. Bouvard and Nicollet made an exten- 
sive series of observations on this spot, for determining the 
amount of hbration and the position of the Moon's axis. The 
selenographic place was found to be in longitude 8^ 47' W. 
and latitude 14^ 27' N. The averao^e altitude of the edo-e of 
the crater appears to be *7600 feet, and the breadth of the same 
a little over twenty-five miles. 

Pico is a remarkably steep isolated rock in the Mare Imbrium, 
and appears particularly bright when view^ed under favorable 
circumstances. The summit rises full 7000 feet above the sur- 
rounding plain. The position of this mountain is in longitude 
9^ 2' E. and latitude 45^ 5' K 

The following Table exhibits the altitude in English feet of 
the principal lunar mountains, calculated from the observations 
of Professor Madler : 



Newton 

Curtius 

Casatus 

Posidonius 

Short . 

Moretus 

Calippus 

Mutus 

Huyghens 

Clavius 

Blancanus 

Kircher 

Hainzel 





Selenographic 


; position 


Altitude in feet. 


Longitude. 


Latitude. 


. 23.800 . 


. 16° E. . 


. 77° S. 


. 22,200 . 


. 3° W. . 


. 67° S. 


. 20,800 . 


. 35° E. . 


. 74° S. 


. 19,800 . 


. 29° W. . 


. 31° N. 


. 18J00 . 


. 10° E. . 


. 74° S. 


. 18 400 . 


. 7°E. . . 


70° S. 


. 18,300 . 


. 10° W. . . 


39° N. 


. 18.300 . 


. 30° W. . 


63° S. 


. 18,000 . 


. 2° E. . 


20° N. 


. 18.000 . 


. 15° E. . 


58° S. 


. 18,000 . 


. 21° E. . 


63° S. 


. 17,600 . 


. 43° E. . 


67° S. 


. 17,500 . 


. 32° E. . . 


41° S, 



72 



THE SOLAR SYSTEM. 



Selenographic position. 
Altitude in feet. Longitude. Latitude. 



Catharina . 


. 17,400 . 


. 23° W. . 


• 17° S. 


Theophilus . 


. 17,300 . 


. 26° W. . 


. 11° S. 


Tycho (W. border) 


. 17,100 . 


. 12° E. . 


. 43° S. 


Picard 


. 17,000 , 


. 54° W. . 


. 14° N. 


Pythagoras . 


. 16,900 . 


. 60° E. . 


. 63° N. 


Werner 


. 16,600 . 


. 8° W. . 


. 28° S. 


Macrobius . 


. 16,200 . 


. 45° W. . 


. 21° N. 



Out of the twenty lunar mountains whose altitudes exceed 
three English miles, or about 16,000 feet, fourteen are situated 
south of the Moon's equator. This hemisphere appears more 
generally mountainous than the northern one. The height of 
the rock to which the name of our illustrious Newton has been 
assigned, is not much less than the heights of some of the 
loftiest summits of the Andes, though the diameter of the 
Earth is to that of the Moon as 3*7 to 1, and consequently a 
mountain of proportionate altitude on our globe would stand 
16i miles above the' surface. 

We subjoin the breadths in Enghsh miles of some of the 
larcrer craters or annular mountains on the Moon's surface, as 
inferred from the observations of Professor Madler. It will be 
remarked that the six broadest cavities are in the third quadrant, 
south of the lunar equator : 





Breadth of 








Crater or Cavity 


Selenographic. 


Name of Mountain. 


in English miles. 


Longitude. 


Latitude. 


Bailly, 


.149 ... 


65° E. 


. . 65° S. 


Clavius, . 


.143 ... 


15^ E. 


. . 58° S. 


Schikard, . 


.127 ... 


55° E. 


. . 44° S. 


Ptolemy, . 


.115 ... 


3° E. 


9°S. 


Schiller, . 


.113 ... 


38° E. 


. . 52° S. 


Phocylides, 


. 96 ... 


56° E. 


. . 52° S. 



Of 148 craters, whose diameters were measured by the 
same astronomer; — 







THE MOON. 




2 were 


1 between 1 


geographical mile and 2 


7 


(( 


2 


geographical miles and 3 


16 


(( 


3 




4 


19 


(C 


4 




6 


17 


i( 


5 




" 6 


18 


u 


6 




7 


11 


it 


7 




« 8 


9 


(( 


8 




9 


12 


a 


9 




10 



73 



And 36 were above 10 geographical miles in diameter. 

The lunar seas^ as they are termed, are thirteen in number, 
the names selected by the latest selenographei-s being as fol- 
low: — 

Mare Anstrale. Mare Nectaris. 

Mare Crisium. Mare Nubium. 

Mare Foecunditatis, Oceanus Procellarum. 

Mare Trigoris. Mare Serenitatis. 

Mare Humboldteanum. Mare Tranquillitatis. 

Mare Humorum. Mare Vaporum. 
Mare Imbrium. 

The Mare Crisium is 280 miles in diameter from N. to S., 
and rather more than 3oO from E. to W. There is a decided 
greenish tinge about it, which is very peculiar. The Mare Im- 
brium is the largest of the dark circular spots on the Moon's 
surface — the measures of Professor Madler giving for the 
breadth, in a meridional direction, 680 miles, and from east to 
west, 750 miles. It is therefore five times larger than the Mare 
Crisium. The Mare Nuhium has a light gray tinge, and is not 
so dusky in appearance as some other spots near it. The Ocea- 
nus Procellarum is by far the largest of the lunar seas, and 
covers a surface of 90,000 square geographical miles. Its pre- 
vailing color is gray, but is not so dark as in the Mare Crisium, 
though more so than the greenish plains of the Mare Humo- 
rum, &c. The Mare Serenitatis is an elliptical spot, having the 

4 



74 THE SOLAR SYSTEM. 

longer axis from S.W. to N.E. ; its average diameter is about 
430 miles, the tinge greenish as in some other cases. The 
Mare Tranquillitatis is of a clear gray color. 

All these so-called seas are covered with annular mountains, 
craters, and rocks. They are best seen under very small opti- 
cal aid. 

Besides the thirteen great seas and oceans, we have the 
Sinus Iridum, or Bay of Rainbows, a most beautiful spot on 
the northern border of the Mare Imbrium, which will be most 
advantageously observed when the Moon is between nine and 
ten days old. At this time, the summits of the semi-circular 
range of rocks inclosing the bay are strongly illuminated, while 
a strong greenish shadow marks the valley at its base. There 
is also the Sinus Medii in the centre of the Moon's disc, the 
Sinus Ostium, with several other bays and lakes of lesser 
note. 

Near the North Pole of the Moon is a mountain rather 
more than 9000 feet high, the summit of which must have 
continual sunshine, while the plain at its base will have an al- 
ternation between day and twilight. The nearest mountain to 
the South Pole is called Malapert, in latitude SY-^^. When 
the Moon is near the first quarter, the neighborhood appears a 
fine line of points of light. The southern polar district is much 
more mountainous and rugged than the opposite one. 

If a lunar atmosphere exists, it must be one of excessive 
rarity and of no great extent, otherwise it would give rise to 
phenomena which could not fail to attract the attention of the 
observer. Astronomers have long held a divided opinion on 
this subject, and it is still a qucestio vexata. The latest selen- 
ographer. Professor Madler, is of opinion that there may be a 
thin atmospheric envelope of variable extent. 

Some authorities adduce an argument in favor of the pres- 
ence of a lunar atmosphere, from a curious appearance occasion- 



THE MOON. 75 

ally noticed when the Moon passes before a star — a phenome- 
non technically known as an occultation. It most frequently 
happens that the star disappears instantaneously on coming in 
contact with the Moon's Hmb, and re-appears as suddenly and 
completely when emerging from behind her disc. But this is 
not invariably the case. It has been remarked that instead of 
vanishing entirely at the moment of contact, the star is some- 
times seen projected on the Moon's disc for several seconds of 
time, and a similar appearance takes place (though, we believe, 
more rarely) before the final emersion from the other limb. 
About twenty years since, a good deal of interest was excited 
among astronomers generally in reference to this matter, and 
some occultations of the bright star in Taurus (Aldebaran) were 
closely watched at the principal European observatories, with 
the view of bringing the question to some satisfactory solution ; 
but the result of these and other more recent observations have 
proved very far from conclusive. To illustrate this by a single 
example. At the Royal Observatory, Greenwich, some of the 
observers perceived nothing unusual either at the immersions or 
emersions of Aldebaran : the star disappeared and re-appeared 
instantaneously. Others, on the contrary, saw it distinctly pro- 
jected upon the Moon's disc for a second or two before it was 
finally hidden behind her ; and these persons were observing 
with similar instruments, and from the same station as the for- 
mer. Professor Powell has suggested that the phenomena may 
be accounted for theoretically on the laws of irradiation ; but 
there still remains the difficulty of explaining why some astron- 
omers should see the stars projected, while others see nothing 
of the kind. Supposing that a lunar atmosphere exists, the 
projections might be ascribed to the refraction of the rays of 
light from the star in passing through it ; and we might even 
go farther, and explain how it happens that they are not always 
noticed, if we admit, with Professor Madler, that the atmos- 



76 THE SOLAR SYSTEM. 

phere is of variable extent, and may occasionally contract with- 
in very small limits. Yet there will still be the same difficulty 
of clearing up the anomaly, that out of a number of practised 
observers, at the same place and time, some should regard the 
immersions and emersions as instantaneous, and others be con- 
vinced of a projection of the star upon the Moon's disc, or of 
its " hanging" for a few seconds upon her Hmb. These facts 
appear to point to the instruments employed, and to the ob- 
servers themselves, for a satisfactory explanation of the whole. 

Several instances are on record where a star, instead of dis- 
appearing finally when first in contact with the Moon's limb, 
has run along it and re-appeared several times, evidently be- 
tween the mountains upon the edge of her disc. On the Tth 
of March, 1794, Professor Koch saw Aldebaran disappear and 
reappear three times, about thirty seconds or so intervening be- 
tween the immersions and emersions. Another observation of 
a similar kind was made by Mr. Riiraker at Hamburg, on the 
19th of February, 1820. A star of the seventh magnitude ap- 
peared to run with extreme rapidity along the summits of the 
mountains on the Moon's edge, by which it was eclipsed from 
time to time. This " magnificent spectacle" continued nearly 
ten minutes, after which the star entirely vanished. 

Occultations by the Moon of the planets Jupiter and Saturn 
are phenomena of great interest, though not of frequent oc- 
currence. 

The existence of active volcanoes upon the Moon is a sub- 
ject which has been a good deal discussed, particularly within 
the last seventy years. HeveHus, when he was engaged upon 
his Selenographia^ remarked that the spot called by him Mons 
Porphyrites^ but now known as Aristarchus, appeared reddish, 
and seemed to burn, or rather to emit flames, whence he con- 
jectured that its nature was similar to that of Vesuvius or 
JEtna, or the other volcanoes upon the Earth's surface. In 1787 



THE MOON, ^J'J 

Sir William Herschel announced that lie had observed three 
volcanoes in actual operation in different parts of the Moon. 
The principal one was situated about 4' from the northern 
limb. It was closely watched on the 19th and 20th of April. 
The diameter of the burning part was three seconds of arc, or 
about three miles. All the adjacent parts appeared to be illu- 
minated by the eruption. The other volcanoes were much 
nearer the centre of the Moon. They exhibited no well-defined 
luminous spots, but resembled "large, pretty faint nebulae, that 
are gradually much brighter in the middle." After the publi- 
cation of these observations, the attention of astronomers gen- 
erally was directed to the subject. Luminous appearances have 
been repeatedly noticed in the dark part of the Moon, when not 
more than three days from conjunction, during the partial ob- 
scuration of her surface in a lunar eclipse, and when her dark 
body has been seen projected on the Sun, during an eclipse of 
that luminary. The bright spot thus observed is almost inva- 
riably upon the Mons Porphyrites of Hevelius, or the Aristar- 
chus of Riccioh, and by far the gi'eater number of observations 
have been made soon after or before new Moon, when this part 
of the disc is far distant from the boundary of the illuminated 
surface, but at the same time in a position to receive a great 
deal of earth-light. Shortly before the lunar eclipse of August 
2, 1822, Flauguergues remarked that Aristarchus seemed, as 
usual, more brilliant than the general surface of the Moon, its 
color being a very decided yellow. As the penumbra ap- 
proached, it appeared grayish, and more so when the spot was 
fully covered by the dark shadow ; but still it was clearly dis- 
cernible, though the spots known as Kejpler and Copernicus 
vanished as soon as they were immersed in the shadow. During 
more than two hours, while the Moon was undergoing eclipse, 
the appearance of Aristarchus was the same ; the light was so 
distinct and striking that an observer might readily suppose he 



78 THE SOLAR SYSTEM. 

saw a lunar volcano. Flauguergues, however, adopts the 
opinion of Mechain, Olbers, and other astronomers, who consider 
this phenomenon due to the illumination of the flat summit of 
Aristarchus by the " lumiere cendree,"* as it is termed ; it is 
probable that this particular spot is of a nature to reflect the light 
more readily than others, which would account for its being so 
repeatedly observed as a brilliant point. The flickering flame- 
like motion which is occasionally remarked can be due only to 
certain conditions of the Earth's atmosphere. When the sky 
is perfectly clear, and the air still, the summit of the mountain 
is seen steadily illuminated ; but under less favorable circum- 
stances, the light has all the w^avering motion which a volcanic 
eruption might be supposed to induce. 

During the total eclipse of the Sun, on the 24th of June, 
1778, Don Antonio de Ulloa saw upon the disc of the Moon a 
luminous point, which was, strangely enough, conjectured to be 
a trough or canal cut through the body of the Moon, through 
which the Sun's light was visible. This is an explanation very 
unlikely to meet with the concurrence of astronomers. It is 
far more probable, as Flauguergues conjectures, that the phe- 
nomenon was owing to the phosphorescence of Aristarchus, par- 
ticularly as a diagram of the position of the brilliant point upon 
the Moon's dark body agi'ees tolerably well with that of the 
spot. 

After what has been here stated, the reader will see that 
there is no absolute necessity to admit the existence of active 
volcanoes upon the Moon in order to account for the appear- 
ances which have been observed upon her disc from the time 
of Hevelius to the present day. It is only necessary to sup- 
pose that, from the peculiarly reflective nature of the spot 

* By the " Lumiere Cendree" is understood the light derived first 
by the Earth from the Sun, afterwards reflected from the Earth to the 
Moon, and finally from the Moon to the Earth. 



THE MOON. 79 

Aristarehus, that part appears brighter than the rest of the lunar 
suiface when the Earth is shining strongly on those regions, 
while, if we admit a kind of phosphorescence in the spot, which 
causes it to emit during darkness the light it had previously 
imbibed during sunshine, we may explain without much diffi- 
culty the brilliant points recorded as having been observed 
when the Moon is immersed in the Earth's shadow, or is seen 
projected upon the Sun. The prevailing opinion amongst as- 
tronomei-s at the present time is consequently adverse to the 
existence of active lunar volcanoes. 

Hevelius was the first astronomer who paid much attention 
to the delineation of the telescopic appearance of the Moon, 
His Selenographia, published in 1647, is a detailed description 
of our satellite, and contains, besides a general chart, about 
forty special charts for different phases, all drawn and engraved 
by himself. The optical power, however, employed in their 
formation vras so small, that they have long since been supersed- 
ed by others bearing greater pretensions to accuracy. Father 
Riccioli of Bologna published a lunar map in 1651, the various 
spots being distinguished by proper names, instead of the 
geographical ones, (fcc, assigned by Hevelius, and this nomen- 
clature has been closely followed, as already remarked, by suc- 
ceeding astronomers. The celebrated Dominic Cassini formed 
a chart of the Moon's surface in 1680, and about the year 
1749, Tobias Mayer of Gottingen published one, which, though 
smaller than Cassini's, is more accurate, and, in fact, the best 
that was in the possession of observers up to the year 1824. 
The indefatigable Schi-oter of Lilienthal was the author of a 
large work, printed in 1791, and entitled " Selenotoijograjphlc 
Fragments ^^^ in which are given special charts of many of the 
principal spots upon the Moon. This work is now become 
somewhat rare. Schroter evinced great perseverance in his ex- 
amination of our satellite ; but it appeai-s he was not always 



80 THE SOLAR SYSTEM. 

fortunate in his description of particular tracts upon the sur- 
face. In 1824, the first part of what was intended to be an 
elaborate work upon this subject was pubhshed by W. G. 
Lohrmann of Dresden. The charts are stated by Madler to be 
very exact, and it is most unfortunate that their appearance 
was interrupted, so that instead of twenty-five maps, as in- 
tended, we are only in possession of four. 

In 1837, the very elaborate and excellent chart of the 
Moon by M.M. Beer and Madler was placed in the hands of 
astronomers, and, from its extent and minuteness of detail, 
echpsed all others. The chart accompanies the fine work of 
M.M. Beer and Madler upon the Moon, in which they have 
given us the results of a most laborious examination of the 
various spots. The large map is three feet in diameter ; but a 
smaller one, of about one foot diameter, was also published, 
and is quite sufficient for recognizing the principal moun- 
tains, &c., exhibited by ordinary telescopes. The larger chart 
loses much of its utility without the accompanying descrip- 
tion. This great work of M.M. Beer and Madler will doubt- 
less be regarded as the standard work upon the Moon for many 
years to come ; and when another examination of the surface 
of equal extent and precision has been made half a century 
hence, it will be most interesting to compare the results with 
those of the German astronomers, as such comparisons may 
lead to conclusions as unexpected as they would prove impor- 
tant in the physical history of our satellite. 

In addition to general and special charts of the Moon, 
models of various craters and chains of mountains have been 
executed with the help of powerful telescopes and microme- 
ter ; and a Hanoverian lady, Frau Hofrathinn Witte, has ex- 
tended this modelling on a small scale to the whole visible 
surface of the Moon. Sir John Herschel gave a description 
of this beautiful model of Madame Witte's at the meeting of 



THE MOON. 81 

the Royal Astronomical Society in November 1845 : only two 
copies are in existence, — one was exhibited on this occasion ; 
the other is deposited in the Royal Museum at Berlin. The 
positions and general outlines of the lunar craters and moun- 
tains were first laid down from Madler's great work upon a 
twelve-inch globe composed of mastic and wax, and the details 
were afterwards filled in by Madame Witte from her own ob- 
servations with a Fraunhofer telescope, placed upon the roof 
of her dwelling-house. In order to exhibit the relative heights 
of the various mountains upon this small globe, they are laid 
down twice as high as they should be in proportion to the 
diameter. It is to be regretted that all attempts to multiply 
copies of this elaborate and beautiful work have hitherto failed. 

A model in plaster of Paris of the region about the crater 
Maurolycus has been constructed by Mr. Nasmyth of Man- 
chester, and is in the possession of the Astronomical Society 
of London, as also a smaller model of the fine crater Eratos- 
thenes and its vicinity. 

The appearance of the heavenly bodies to the inhabitants 
of the Moon, if any, are widely different from those we witness 
ourselves. The Earth appears to them a great globe more 
than 2^ in diameter, and, so to speak, is a fixed object in their 
heavens, only altering her place by the amount of the libration, 
or oscillating to and fro in a space of 15^ 8^ of longitude, and 
13^ 6' of latitude. The stars and planets, therefore, pass behind 
her, and occulta tions of these objects by the Earth must be in- 
teresting phenomena to the lunarians. Our globe, moreover, 
reflects a vast amount of hght for their benefit, and exhibits to 
them all the varied phases w^hich are . presented to us in the 
course of a lunar month, but in inverse order. Thus, when the 
Moon is at the first quarter, the Earth will be in her last quar- 
ter, when our satellite is full to us, we are new to them, or 
they have the Earth in conjunction with the Sun. These re- 

4* 



82 THE SOLAR SYSTEM. 

marks apply only to those parts of the lunar surface which are 
turned towards our globe, for the inhabitants of the opposite 
side never see the Earth at all, and those who are located on 
the apparent borders of the kinar disc only now and then ob- 
tain a glimpse of it in their horizon, for which they are in- 
debted to the librations in longitude and latitude which we 
have already noticed. 

At the lunar equator the solar day is of a constant length, 
and about equal to 354h. 22m., or 14d. 18h. 22m. of our 
mean solar time. At a latitude of 45^, the longest day is 
357h. 19m., and the shortest 351h. 26m. The difference, of 
course, increases as we approach the poles, and in 88^ of lati- 
tude the longest day is 449h. 28m., or 18d. I7h. 28m., and 
the shortest 259h. 16m., or lOd. 19h. 16m. of our time. The 
mean length of a day at the Moon is equal to half a synodic 
revolution round the Earth. On the summit of the mountain 
known as Huyghens, which, according to the measurement of 
Professor Madler, has an altitude of 18,000 feet, the mean 
length of a day exceeds by 18h. the average length on the 
surface at that latitude, whereas the loftier summit of Chim- 
borazo, on our own globe, only experiences an increase of 20m. 
on the mean length of a day on the plains. In those parts of 
the Moon which remain always invisible to us, night must be 
totally dark, — no earthshine can reach them, and the brightest 
objects in their heavens will be the planets Mars and Jupiter, 
which can afford no more light to the lunarians than they do to us. 

Pytheas, of Marseilles, who lived about 330 b.c, is said to 
have been the first to point out the influence of the Moon upon 
the ebb and flow of the waters of the ocean, yet his theory seems 
to have obtained little credence at the time, and the astronomer 
gained nothing by its divulgement Kepler claims priority in 
the production of a treatise upon this subject, in which he 
showed that the action of both Sun and Moon is to be con- 



THE MOON. 83 

sidered ia accounting for the phenomenon, but it was not till 
JSTewton's Principla appeared that the modus operandi was 
explained. Owing to the much greater proximity of the 
Moon, her infiuenca preponderates over that of the Sun, though 
the latter has still sufficient power to bring about a consider- 
able variation in the heights of the tides according to his 
position with regard to the Moon. As our satellite, in the 
course of a lunar day (about 24h. 50m.), passes successively 
over the meridian of every place upon the Earth's surface, the 
waters of the ocean are drawn after her by the attractive in- 
fluence exercised upon them, the greatest wave arriving on any 
particular meridian a short time after the Moon has passed 
over it, since her action is not instantaneous. So also the Sun. 
in the course of a solar day, produces a much smaller wave, 
which will coincide or otherwise with the lunar one, according 
to his position in respect to the Moon. 

Now if we bear in mind that the Moon not only attracts 
the waters upon the surface of our globe, but has a similar in- 
fluence upon the Earth itself, it will not be very difficult to 
account for what at first sight may appear a very singular 
phenomenon — that the tidal wave is produced simultaneously 
upon those parts of the Earth which are furthest from the 
Moon, or have her at a maximum depression below their hori- 
zon, as well as upon those parts which have her nearest to 
them, or on their visible meridian ; in other words, soon after 
our satellite has culminated at any place, whether on the upper 
or lower meridian, the waters in the neighborhood are most 
elevated above their ordinary level. It is then from the cir- 
cumstance of the Moon having a more powerful influence upon 
the Earth itself than upon those seas which are farthest from 
her that the waters are left behind, so to speak, to the amount 
of the difference in the attraction of the Moon upon them, and 
upon the intervening land. 



g4 THE SOLAR SYSTEM. 

The interval elapsing between two maxima of the tidal 
wave at any place, is rather more than twelve hours, or about 
half the lunar day of 24h. 40m. Hence, in the course of a 
day, we have high water and low water twice. There are sev- 
eral circumstances tending to vary the amount of elevation 
of the waters, as the change in the distances of the Sun and 
Moon from the Earth, and in their declinations at different 
times. The tides which occur at the syzigies are usually called 
the spring tides, and those at the quarters the neap tides. The 
highest tide takes place generally when the Moon is in perigee 
and on the equator. Local winds, and other causes of a sim- 
ilar kind, tend greatly to throw uncertainty upon any deduc- 
tions relating to the height of the tides ; but their general 
theory is now well understood, — the Astronomer Royal, Dr. 
Whewell, Sir John Lubbock, and others of our own country- 
men having devoted much time and labor to the subject. 



CHAPTEE YI. 

ECLIPSES OF THE SUN AND MOON. 

THE eclipses of the Sun and Moon, particularly of the former, 
are amongst the most imposing phenomena of the heavens, 
and have been observed and studied from the remotest ages of 
- antiquity. 

The term eclipse is applied in the case of an obscuration of 
either of these luminaries. When the Moon, in the course of 
her monthly revolution round our globe, comes precisely into a 
line with the Earth and Sun at opposition, she must be im- 
mersed in the shadow of the former, and, as her light is reflected 
from the Sun, an eclipse or darkening of her disc must take 
place. This eclipse may be total or partial only, according as 
the Moon passes centrally or otherwise through the Earth's 
shadow ; in other words, according to her distance from the 
node at the time of syzigy. If this distance exceed a certain 
number of degrees no eclipse can take place ; within another 
known hmit a partial obscuration may occur ; and, if the argu- 
ment of latitude is still less, the Moon must be entirely immei-sed 
in the shadow about the nodal passage, and a total eclipse will 
be the result. 

Again, if the Sun, Moon, and Earth are in the same line at 
conjunction (^.^., with the Moon between the Sun and Earth), 
or nearly so, the dark body of the Moon will intervene between 
the Earth and Sun, and cause a total or partial obscuration of 
the latter, or, as occasionally happens, if the Moon passes ex- 



86 THE SOLAR SYSTEM. 

actly between these two bodies at syzigy, but so far from the 
Earth as to have a less apparent diameter than the Sun, a phe- 
nomenon termed an annular ecHpse will take place, a small 
portion of the bright surface of the Sun appearing like an annu- 
lus or ring round the dark body of the Moon. 

It is evident, therefore, that the phenomenon called an 
eclipse of the Moon, is produced by causes entirely different 
from those which operate in an eclipse of the Sun. A solar 
eclipse would be more properly termed an occultation of the 
Sun by the Moon. 

If the plane of the Moon's orbit exactly coincided with the 
ecliptic, there would inevitably happen an eclipse of the Sun 
and one of the Moon in every lunation, but as its inclination 
thereto exceeds 5^, the Moon's latitude at the time of conjunc- 
tion or opposition will frequently be so great as to cause her 
to pass above or below the limits within which eclipses may 
happen, and, consequently, none will take place. The occur- 
rence of these phenomena depends, therefore, upon the distance 
of the Moon from her node, or her latitude in conjunction and 
opposition. 

According to the latest tables of the Sun and Moon formed 
by Carlini, Damoiseau, and Burckhardt, in order that an eclipse 
of the Sun may take place, the greatest possible distance of the 
Sun or Moon from the true place of the ascending or descend- 
ing node of the Moon's orbit is 18^ 36' ; and an eclipse is im- 
possible if the Moon's latitude exceed 1° 34' 52'', while, if it be 
less than 1° 23' 15", an eclipse must necessarily take place ; 
between these limits the occurrence of one at any station is 
doubtful, but depends upon the parallaxes and apparent semi- 
diameters of the two bodies at the moment of conjunction. 

Employing the same tables, it is found that an eclipse of 
the Moon may occur if her distance from the true place of the 
node at the time of opposition does not exceed 12° 24': the 



ECLIFSES OF THE SUN AND MOON 87 

greatest possible latitude is 63' 45'' : if it be less than 51' 57'^ 
an eclipse is certain ; between these limits it is doubtful, but de- 
pends, as in the former case, upon the actual value of the semi- 
diameters and horizontal parallaxes. 

The greatest number of eclipses that can happen in any one 
year is seven, and, of these, five may be solar and two lunar, or 
three solar and four lunar. The average number is four, and 
the least two : in the last case both will be solar. The varia- 
tion in the number of eclipses is easily explained. During the 
time the Earth occupies in passing through the solar ecliptic 
limits, a new Moon must necessarily take place, and therefore a 
large solar eclipse ; but, at the full Moon immediately prece- 
ding, it may happen that the Earth had not got within the 
lunar ecliptic limits (which are less than those of the Sun nearly 
in the proportion of two to three), while, at the next full Moon, 
our globe may have passed beyond them, which accounts for 
the n on- occurrence of any lunar eclipse that year. Again, with 
regard to the greatest number of eclipses, we observe that 
twelve lunations occupy about 354d. 36m., which is nearly 
eleven days less than the mean length of the solar year. Con- 
sequently, if an eclipse — say one of the Sun — should happen 
before the 11th of January in any particular year, and there 
should occur at that and the following node two solar and one 
lunar eclipse at each, then, at the twelfth lunation, which will 
take place before the end of a solar year, the Earth, in conse- 
quence of the retrograde movement of the Moon's line of nodes, 
may have got within the solar ecliptic limits, and a fifth solar 
echpse may occur within the year. If we had supposed the 
first eclipse to be lunar, then we should have three of the Sun 
and fom- of the Moon, or seven in all, as above remarked. 

We have seen that the eclipses of the Sun and Moon de- 
pend upon the position of the latter luminary, in respect to her 
node at the time of new and full Moon, and it is therefore evi- 



88 THE SOLAR SYSTEM. 

dent that if any cause should operate to bring about a similar- 
ity in their conditions after the lapse of certain intervals, the 
eclipses will become cyclical phenomena. It so happens that 
such a cause does operate, arising from the near commensura- 
bihty of 223 mean synodical revolutions of the Moon on which 
the phases depend, and 19 synodical revolutions of her nodes, 
the former extending to 6585'32 days, and the latter to 6585*78 
days. The agreement, it will be observed, is not exact, other- 
wise a recurrence of all echpses under the same condition must 
take place in every period of rather more than eighteen yeai-s, 
but the difference, amounting to less than half a day, is so small 
that it is found that eclipses do occur in something like regular 
order after the completion of nineteen synodic revolutions of the 
Moon's nodes. A knowledge of this fact, as far as regards the 
length of the interval, may perhaps have enabled the ancient 
astronomers to foretell the occurrence of a great eclipse, since it 
is quite certain they did so in more than one instance before 
the true nature of eclipses was understood, and the eighteen 
year cycle is said to have been known to the Chaldeans under 
the name of saros. 

ECLIPSES OF THE SUK. 

A solar eclipse may he partial, — that is, a portion only of 
the dark body of the Moon may intervene between the Sun and 
the observer on the surface of the Earth ; it may be total, if the 
apparent diameter of the Moon exceed that of the Sun, and the 
former body passes nearly centrally before that of the latter ; or 
it may be annular, when the Sun's apparent diameter is greater 
than that of the Moon, traversing his globe as before. If the 
centres of Sun and Moon exactly coincide in the latter case, the 
eclipse is said to be central and annular, the Sun appearing for 
a moment only as a brilliant ring or annulus on the dark ground 
of the heavens, of uniform breadth in every part. Total eclipses 



ECLIPSES OF THE SUN, 89 

are perhaps the most grand and imposing, and central and an- 
nular eclipses amongst the most beautiful of celestial phenom- 
ena. The former have been viewed in all ages with awe and 
astonishment ; the gradual diminution in the light of the gi'eat 
ruler of the day, its sudden total extinction, the almost super- 
natural appearance of the heavens and surrounding objects 
during total darkness, the visibility of the stars in daytime, and 
other phenomena accompanying a large eclipse, are all calcu- 
lated to inspire the mind v/ith feelings of reverence for the 
Great Being whose power they proclaim, while they must at the 
same time impress upon it a feeling of admiration for that sub- 
lime science by the laws of which man is able to foretell the 
occurrence of eclipses, not to the day or the hour only, but to 
the very minute, — not merely a few years beforehand, but for 
centuries in advance. 

Total eclipses of the Sun in any particular locality are phe- 
nomena of very rare occurrence. Thus, in London, none had 
been observed between the 20th of March, 1140, and the 22d 
of April, 1715, though in the interval the shadow of the Moon 
had repeatedly passed over other parts of Great Britain.* The 
next total eclipse visible in England will take place on the morn- 
ing of the 19th of August, 1887, and large eclipses will happen 
on March 15th, 1858, March 6th, 1867, December 22d, 1870, 
and May 28th, 1900. 

In order to give the reader some idea of the appearances he 
may expect to witness in a total, or very large eclipse of the 
Sun, we shall here particularize some of the most remarkable 
phenomena to which astronomers have drawn attention, while 
at the same time we- describe the more usual characteristics of a 
solar eclipse. And, in the first place, it may be observed, that 
the determination of the precise time when the hmbs of the 
Sun and Moon appear to touch each other, or, as it is techni- 
* According to Dr. Halley. 



90 THE SOLAR SYSTEM. 

cally called, the moment of first contact, is one of great impor- 
tance, inasmuch as the longitudes of places may be thereby ascer- 
tained, from comparative observations, with a high degree of 
accuracy. Careful and practised observers will seldom differ 
more than two or three seconds, especially if the instrumental 
means employed at different stations are pretty nearly the same 
as regards optical capacity. The angle from the apparent 
north point of the Sun's limb, where the contact with the 
Moon's dark body takes place, is always calculated beforehand 
from the solar and lunar tables, and the astronomer w^ill have 
his eye directed to the precise spot some minutes previous to the 
computed time of the commencement of the eclipse. It has 
happened once or twice during these preparatory moments that 
an appearance resembhng a faintly illuminated limb of the 
Moon has been perceived near the border of the Sun, which 
would tend to establish the visibility of a portion of the lunar 
disc as a bright object, before the actual contact of limbs : this 
has been noticed in England and America during the present 
century. The occurrence of first contact is sometimes indicated 
by the appearance of one or more prominent points upon the 
Sun's limb, possibly attributable to the mountains on the appa- 
rent edge of the Moon. As the echpse progresses, flashes of 
light are occasionally remarked upon the lunar disc, and in one 
instance a solar spot, situated near the Moon's edge, seemed to 
be suddenly illuminated. Luminous appearances of a more 
permanent character have been witnessed upon her surface, 
either in form of a bright spot or a narrow stream of light. 
The visible edge of the Moon's disc has frequently been observed 
to project beyond the solar cusps, in the first instance by Heve- 
lius, and many times during the last and present centuries. As 
the total obscuration approaches, the remaining portion of the 
Sun's illuminated disc gradually changes color, becoming either 
reddish or of a fainter color than before. One observer of the 



ECLIPSES OF THE SUN. 91 

total eclipse of July 7, 1842, states that the thin crescent of 
light was suddenly changed to a line of luminous points, which 
appeared to wave fi-om the extremities to the centre of the 
crescent, like " a device in gas swept over by a strong breeze," 
and similar appearances have been more or less perfectly ob- 
served by others. The diminution of light, though gradual, is 
not usually very great until a few minutes previous to the total 
obscuration of the Sun, when the color and general appearance 
of terrestrial objects change rapidly, and an unnatural gloom 
prevails. As the last trace of the Sun vanishes, darkness in- 
stantaneously supervenes, and this sudden transition is described 
by all who have been fortunate enough to witness it as a most 
imposing, nay, awe-inspiring phenomenon. 

The Moon's surface is sometimes partially illuminated by a 
faint doubtful kind of light during the totality, and on one 
occasion the spots were distinctly observed by Vassenius at 
Gottenberg. M. Arago and Mr. Airy also bear witness to the 
uniform illumination of the disc, but could see no inequalities 
of light, either of the nature of a dark tract or bright spot : 
hence it would appear that this lumiere cendree, as it is termed, 
which is reflected from the Sun to the Earth, afterwards from 
the Earth to the Moon, and finally from the Moon to the Earth, 
is of variable intensity, and sometimes is entirely overpowered 
by what M. Arago calls the lumiere aimosphei'ique. The most 
beautiful appearance peculiar to a total eclipse is that of the 
corona^ or ring of hght of variable extent, suiTounding the dark 
body of the Moon, and very closely resembhng the '* glory" 
with which painters encircle the heads of saints. It was first 
generally remarked during the eclipse of 1715, and was par- 
ticularly described by Halley, LouvUle, kc. ; but a similar lumi- 
nous ring is referred to by observers at a much earlier period. 
It must be borne in mind that we are at present treating only 
of total eclipses, and this corona has therefore no connection 



92 THE SOLAR SYSTEM. 

with the annuluSj which is formed in a central and annular 
eclipse, to be noticed further hereafter. In July 1842, the 
attention of astronomers was especially directed to the appear- 
ance of the corona, Mr. Airy, who made a journey to Turin 
for the purpose of witnessing the great eclipse, describes it as 
a ring of peach-colored hght, and possibly with somewhat of a 
radial appearance, though not sufficiently marked to interfere 
with the general annular structure. Mr. Baily, on the contrary, 
says the corona had the appearance of brilliant rays, and that 
he was unable to trace the well-defined shape of a ring at the 
outer border. The rays were vivid and flickering. At Mont- 
pellier, many persons suspected a rotatory motion of the coro- 
na^ and M.M. Otto Struve and Schidlofsky, who observed the 
eclipse at Lipesk in Russia, tell us it was always in a state of 
violent agitation. Neither Mr. Airy nor Mr. Baily remarked 
any material deviation from a uniform breadth in the ring ; but 
at various stations in France, and likewise by one observer at 
Milan, long jets of hght, called aigrettes by M. Arago, were 
particularly noticed. Persons differed much in their estima- 
tions of the breadth of the corona : some considered it hardly 
more than one eighth of the Moon's diameter, or about four 
minutes of arc, while others traced the streams of light three 
or four degrees from the limb of the Moon. These discordan- 
ces must be in some measure due to atmospherical conditions, 
whatever be the true cause of the appearance of a corona. 
The color of the light was equally the subject of doubt. Mr. 
Airy, as we have seen, thought it was peach-colored : Mr. Baily 
says it was perfectly white — and other observers at Narbonne, 
Lipesk, &c., were of the same opinion. M.M. Laugier and 
Mauvais, at Perpignan, judged the aureola to be yellowish, as 
viewed with the telescope, but colorless to the naked eye ; and 
Professor Majocchi, at Milan, says it appeared to him white, 
" with a tendency to yellow." The intensity of the light at 



ECLIPSES OF THE SUN. 93 

Lipesk is stated to have been such that the eye was scarcely- 
able to support it ; yet at Milan, Vienna, Perpignan, and vari- 
ous other places in France, Italy, and Germany, it was pre- 
cisely similar to that of the Moon. M. Arago suggests that 
the much greater altitude of the Sun at the total phase above 
the horizon of Lipesk than at Perpignan, and other stations in 
the south of Europe, may possibly serve to explain, partially at 
least, this wide difference. The Sun was nearly 30° higher at 
Lipesk than at Perpignan. The precise time of the first for- 
mation and final disappearance of the corona is also variously 
given by astronomei*s who witnessed this eclipse. Generally, 
however, it is stated to have been visible some few seconds be- 
fore the extinction of the Sun's light, and after its re-appear- 
ance. At Montpellier the white aureola was perceived five or 
six seconds before totality came on, and about the same time at 
Salon, Marseilles, Pa via, &c. At Alais, where the echpse was 
aot quite, though nearly, total, the corona w^as distinctly seen. 

We have vet to mention another most remarkable and 
beautiful phenomenon, which was witnessed by several eminent 
astronomers during the total echpse of 1842. It consisted of 
rose-colored flames or prominences at various parts of the Moon's 
limb, while the Sun was entirely covered. At Vienna, Pro- 
fessor Schumacher remarked three of these protuberances which 
continued steadily visible, without the flickering pecuhar to the 
corona. They were of a rose-red color, and between one and 
two minutes in altitude. 

Mr. Airy, from the Superga, near Turin, saw three of these 
small flames, of a full lake-red, and brighter than the rest of 
the ring or corona : the distance between the fii-st and third 
was about 40° on the Moon's limb. They w^ere visible without 
the telescope shortly before the re-appearance of the Sun. Mr. 
Baily, at Pavia, also saw three protuberances " apparently ema- 
nating from the circumference of the Moon, but evidently 



94 THE SOLAR SYSTEM. 

forming a portion of the corona." Their color was judged to 
be red, with a tinge of hlac or purple. M. Arago, at Perpig- 
nan, saw two only on the northern limb, rose-colored on the 
whole, but greenish blue in some points. M. Mauvais, at the 
same place, witnessed the gradual increase of a small reddish 
spot to conspicuous " mountains," perfectly well defined, but 
not of a uniform color. They might be compared to the snowy 
tops of the Alpine mountains illuminated by the setting Sun. 
There were three distinct prominences, the last of which be- 
came perceptible about Im. 10s. after the Sun had disappeared. 
Similar phenomena were recorded by observers at Toulon, 
Marseilles, Montpellier, Narbonne, and other places in France. 
At Lipesk, in Russia, M. Schidlofsky saw the rose-colored 
mountains, and also remarked that a considerable portion of 
the Moon's limb was strongly tinged with red. At Padua 
many persons saw the " flames" with the naked eye. Professor 
Santini noticed two " pyramids of fire," which he thought of a 
strong violet hue. M. Gonti saw them long after the Sun had 
reappeared. M. Biela, observing at the same place, counted 
three ^ and agreed with the distinguished astronomer Santini as 
to their purple color. On the first glimpse of the Sun, these 
points seemed to run into one narrow border, and vanished 
altogether after a few seconds. 

Similar phenomena to those we have described, before the 
time of totality comes on, are also remarked as the eclipse 
diminishes. 

We have confined this brief account of the principal phe- 
nomena of a solar eclipse, in a great measure, to that which 
took place in July, 1842, because it is the latest of which par- 
ticulars are fully pubhshed, and was observed by many of the 
most experienced astronomers of the day. It is, however, to 
be remarked, that very few novel appearances were noticed, the 
relations of previous eclipses containing references to most of 



ECLIPSES OF THE SUN. 95 

the phenomena recorded in that year. Thus, the corona sur- 
rounding the Sun and Moon during totality is mentioned by 
several eye-witnesses of the eclipses of IYO6, 1715, 1Y78, 
1806, (fee. : its color in 1706 was golden, in 1715 it appeared 
of a pearl-white, tinged with the hues of the rainbow, accord- 
ing to Dr. Halley, and on other occasions, it has been described 
as reddish, pale yellow, and peach-colored. A flickering or 
unsteadiness of the corona, giving the idea of a whirling mo- 
tion, was remarked as early as the year 1628, and also in the 
eclipses of 1706, 1778, and 1816. The rose-colored flames or 
prominences, projected on the light of the corona, were seen 
(Hiring the total eclipse of May 3d, 1733, by Vassenius, at 
Gothenburg, who has left us a very clear account of them. 
They were three or four in number, of a reddish tinge ; the 
most conspicuous one occupying a position on the Moon's limb 
about midway between the north and west points, or, as we 
should now say, at an angle of position of about 315^. Two 
other observers of the same eclipse witnessed a similar appear- 
ance, though less comj^letely. Possibly the words used by 
Juhus Firmicus, in reference to the total eclipse of a.d. 334, 
July 17, at Constantinople, may apply to the so-called "red 
flames" of modern astronomers. 

During the total obscuration of the Sun, stars of the first 
and second magnitude, and the brighter planets, become con- 
spicuous in the heavens, and on some occasions stars of a fainter 
class have been detected when the atmospheric circumstances 
happened to be unusually favorable. During the total eclipse 
of 1842, the planets Mars and Yenus were distinctly visible at 
many stations in Italy and Germany, and Mercury was percep- 
tible, according to one observer, in Russia. The bright stai*s, 
Capella, Aldebaran, Betelgeux, Castor, and Pollux, were 
amongst those noticed in different places. In some localities, 
as many as ten were counted. The color of the sky, and of 



96 THE SOLAR SYSTEM. 

objects surrounding the observer, is frequently recorded to have 
changed in a remarkable manner, as the time of total extinc- 
tion of the Sun's light approached. So early as a.d. 840, 
this circumstance is mentioned. Dr. Halley, in describing the 
great eclipse of 1715, says, as the totality came on, the color 
of the sky changed from its usual azure blue to a livid purplish 
hue, and the darkness of the eclipse was attended by a chill 
and damp of which everyone was sensible. During the eclipse 
of July 1842, much attention was paid to the natural phenom- 
ena accompanying it, but observers vary a good deal in their 
descriptions. In France the color of surrounding objects be- 
came yellowish, or of a light olive tinge, and in a sea-horiz«i 
a broad band of an orange color was formed. At the com- 
mencement of totality, the figures of persons standing near 
assumed a pale cadaverous aspect according to some authori- 
ties, while others describe them as greenish. In Italy, gener- 
ally a greenish hue covered the whole landscape at this mo- 
ment, gradually changing to violet as the darkness deepened. 
At Nov are, the heavens, up to an altitude of 50°, were of a 
rosy tinge, like that of the Aurora Borealis. One observer 
describes the faint illumination of objects in his neighborhood 
as resembling that afforded by ignited spirits of wine. While 
the vast plains surrounding Milan presented a deep green hue, 
at Cremona the landscape seemed as though it were illumina- 
ted by a Bengal light. These varied accounts are probably 
to be ascribed in some degree to optical causes depending 
on the observei's themselves. 

During the eclipse of 1842, nearly the whole population of • 
some of the principal cities of southern France and Italy, which 
were upon the central line, turned out to view the rare phe- 
nomena of a total deprivation of the Sun's light in the day- 
time. At Pavia, Mr. Baily says, " there was an universal 
shout which made the welkin ring," at the conclusion of the 



ECLIPSES OF THE SUN. 97 

eclipse ; and M. Arago, who observed at Perpignan, says nearly 
twenty thousand persons covered the terraces, ramparts, and 
other eminences about the place, and that an astounding shout 
from this multitude announced the extinction and reappearance 
of the Sun's rays. At Milan, Padua, &c., the excitement was 
equally great. " Long live the astronomers !" was the cry in 
the former city, when the rose-colored flames burst forth on the 
bi-ight ground of the corona during the total obscuration. Two 
hundred years previously, many of the inhabitants of Paris hid 
themselves in caves on the mere announcement of an eclipse of 
the Sun, which was total in that city !* 

The sudden extinction of the Sun's light in total eclipses is 
not without its effects upon the animal creation, and naturalists, 
who have confined their attention to terrestrial observations 
during the totahty, relate some curious instances of the alarm 
and astonishment exhibited, not only by the more sagacious 
animals, but by birds, and even insects. In July, 1842, in the 
south of France, horses attached to vehicles came to a decided 
stand, and no exertions of their drivers, though backed by the 
whip, could induce them to proceed until the Sun had again 
appeared.f Cattle in the fields congregated together imme- 
diately after total darkness came on, as if in apprehension of an 
attack. Dogs, in particular, appear to have been sensible of 
some unnatural event, howling piteously during the deprivation 
of the Sun's rays, or hastily seeking some place of safety. At 
Montpellier the swallows, which were numerous before the com- 
mencement of the total phase, suddenly disappeared until it 
had formed, and in one place it is said a great number of birds 

* Arago — " Annuaire du Bureau des Longitudes"— 1846 = 
t An exception must be made in respect of the horses employed 
in the public diligences, which, singularly enough, betrayed no signs 
of alarm, but, as M. Arago observes, " paid as little attention to the 
phenomenon as the railway locomotives." 

5 



98 THE SOLAR SYSTEM. 

fell upon the earth. That this circumstance has happened in 
previous eclipses there can be no doubt : it is distinctly men- 
tioned by Clavius as having occurred at Coimbra during the 
total eclipse of August 21, 1560, when the gloom is said to 
have been greater than that of night. The stars were distinctly 
visible, and the birds, " mirahile dictu^ fell from the air dead 
upon the earth from fright at so horrible a darkness." In 1842, 
the birds in the trees near Lodi suddenly ceased singing at the 
moment when the total obscuration came on ; but M. Piola 
says he did not notice that any of them fell, though he was sta- 
tioned under one of the trees at the time. M. Arago relates an 
instance where three linnets were placed in a cage outside a 
window early on the morning of the eclipse, and on examina- 
tion, after the phenomenon, one of the three was found to be 
dead, having probably killed itself by striking against the bars 
of the cage in a moment of terror. At Milan the bees quitted 
their hives in great numbers soon after sun-rise, but returned 
to them in haste immediately the last rays j:)f the Sun had van- 
ished. 

The last total eclipse of the Sun, on the 28th of July, 1851, 
was observed under very favorable circumstances by many Eng- 
lish astronomers in Norway and Sweden. The author observed 
near the town of Engelholm, about eighteen miles from Hel- 
singborg, on the Sound, where the eclipse was total for about 
Im. 40s. The moment the Sun went out, the corona appeared ; 
it was not very bright, but this might arise from the interfer- 
ence of an extremely light cloud of the cirrus class, which over- 
spread the Sun at the time. The corona was of the color of 
tarnished silver, and its light seemed to fluctuate considerably, 
though without any appearance of revolving. Eays of light, 
the aigrettes, diverged from the Moon's limb in every direction, 
and appeared to be shining through the light of the corona. 
In the telescope many rose-colored flames were noticed ; one 



ECLIPSES OF THE SUN. 99 

far more remarkable than the rest on the western limb, could 
be distinguished without any telescopic aid ; it was curved near 
its extremity, and continued in view /ot^r seconds after the Sun 
had disajyj^eared, i.e., after the extinction of Bailyh heads ^ which 
phenomena were very conspicuous in this eclipse, particularly 
before the commencement of the totality. In this case they 
were clearly to be attiibuted to the existence of many moun- 
tains and vallevse alono^ the Moon's edo'e, the Sun's lio;ht shin- 
ing through the valley and between the mountain ridges, so as 
to produce the appearance of luminous drops or beads, which 
continued visible some seconds. The color of the '' flames" was 
a full rose-red at the borders, gradually fading off towards the 
centres to a very pale pink. Along the southern limb of the 
Moon, for forty degrees or upwards, there was a constant suc- 
cession of very minute rose-colored prominences, which appeared 
to be in a state of undulation, though without undergoing any 
material change of form. An extremely fine line, of a deep vio- 
let color, separated these prominences from the dark limb of the 
Moon. The surface of our satellite during the total eclipse, 
was purplish in the telescope ; to the naked eye it was by no 
means very dark, but seemed to be faintly illuminated by a 
purplish-gray light of uniform intensity on every part of the 
surface. 

The aspect of nature during the total eclipse, was grand 
beyond description. A diminution of light over the Earth 
was perceptible a quarter of an hour after the beginning of the 
echpse, and about ten minutes before the extinction of the Sun 
the gloom increased very perceptibly. The distant hills looked 
dull and misty, and the sea assumed a dusky appearance, like 
that it presents during rain. The day-light that remained had 
a yellowish tinge, and the azure blue of the sky deepened to a 
purplish-violet hue, particularly towards the north. But, not- 
withstanding these gradual changes, the observer could hardly 



100 THE SOLAR SYSTEM. 

be prepared for the wonderful spectacle that presented itself 
when he withdrew his eye from the telescope, after the totality 
had come on, to gaze around him for a few seconds. The 
southern heavens were then of a uniform purple-gray color, the 
only indication of the Sun's position being the luminous corona, 
the light of which contrasted strikingly with that of the sur- 
rounding sky. In the zenith, and north of it, the heavens 
were of a purplish-violet, and appeared very near, while in the 
north-west and north-east broad bands of yellowish-crimson 
light, intensely bright, produced an effect which no person who 
witnessed it can ever forget. The crimson appeared to run 
over large portions of the sky in these directions irrespective 
of the clouds. At higher altitudes the predominant color was 
purple. All nature seemed to be overshadowed by an un- 
natural gloom, the distant hills were hardly nsible ; the sea 
turned lurid red, and persons standing near the observer had 
a pale livid look, calculated to produce the most painful sensa- 
tions. The darkness, if it can be so termed, had no resem- 
blance to that of night. At various places within the shadow, 
the planets Venus, Mercury, and Mars, and the brighter stars 
of the first magnitude, were plainly seen during the total 
eclipse. Venus was distinctly visible at Copenhagen, though 
the eclipse was only partial in that city ; and at Dantzic she 
continued in view ten minutes after the Sun had reapjDeared. 
Animals were frequently much affected. At Engelholra a calf 
which commenced lowing violently as the gloom deepened, 
and lay down before the totality had commenced, went on feed- 
ing quietly enough very soon after the return of daylight. 
Cocks crowed at Helsingborg, though the Sun was only hidden 
there thirty seconds, and the birds sought their resting-places 
as if night had come on. 

One of the most famous eclipses of the Sun recorded in 
history, is that which put an end to the war between Cyaxeres, 



ECLIPSES OF THE SUN. 101 

King of tlie Medes, and Halyattes, King of the Lydians, de- 
scribed by Herodotus, and said to have been predicted by 
Thales, of Miletus, the celebrated pbilosopber. The contend- 
ing armies were so alarmed at the sudden change of daylight 
into total darkness, that they threw down their arms and con- 
cluded a peace upon the spot. The precise time of this in- 
teresting event has long been disputed by chronologists, and 
the solar eclipses of b.c. 606, 592, and particularly that of 
584, have been fixed upon as the phenomenon mentioned by 
Herodotus ; but since the improvement of the tables of the 
Sun and Moon during the earlier part of the present century, 
several astronomers have occupied themselves in the inquiry ; 
and our countryman Mr. Baily, Professor J. Oltmanns, and 
M. Saint Martin, have pointed out an eclipse on September 
30, B.C. 609,^ which they consider to have been the one in 
question. According to the Tables it was total in Lydia, 
where the battle was fought, and the description left us is only 
reconcilable with the phenomena of a total eclipse, annular 
ones being evidently incompetent to produce entire darkness. 
The central phase happened about four hours after sunrise. 
Recent investigations, however, have shown that the secular 
movement of the Moon's node used in the tables is erroneous, 
to the amount of more than \W and this would cause a con- 
siderable difference in the position of the node in b.c. 609, and 
might very possibly throw the path of the Moon's shadow be- 
yond the probable position of the contending armies. "We 
think it is likely that a new calculation, involving the late cor- 
rections to the lunar elements, will show that the eclipse of 
Thales took place in b.c. 584, and not- in the year assigned by 
the above astronomers. This is an interesting question, and 
deserves the attention of some skilful computer. 

Another total eclipse of the Sun, supposed to have been 
* Reckoning according to the manner of astronomers. 



102 THE SOLAR SYSTEM. 

that of August 3, b.c. 430, had its efifect upon terrestrial 
affairs, and but for the intervention of Pericles, would have 
seriously interfered with the expedition undertaken by the 
Athenian fleet against the Lacedemonians. The date answers 
to the second year of the eighty-seventh Olympiad, the first 
of the Peloponnesian war. Thucydides, who was contem- 
porary, says the stars were seen in the middle of the day. Ac- 
cording to Plutarch, in his Life of Pericles, the commander of 
the vessel was greatly alarmed at the darkness, which took 
place as he was on the point of setting sail ; but the Athe- 
nian General was luckily able to explain the phenomenon, 
w^hich he illustrated in a manner that seems to have allayed 
the feai-s of the captain, and so prevented the delay of his 
expedition. 

The total eclipse, known as that of Agathocles, which oc- 
curred on the loth of August, b.c. 309, was also investigated 
by Mr. Baily. In this case neither the date nor the locality 
were open to much uncertainty, and the phenomenon conse- 
quently appeared to him to afford a favorable opportunity for 
checking the solar and lunar tables, instead of using them as 
the means of settling the date and limits of the eclipse. Dio- 
dorus Siculus says the stars were seen, so that no doubt can 
exist as to the totality of the Eclipse ; but Mr. Baily found an 
irreconcilable difference between the tables and the historical 
statement, a space of about 180 geographical miles appearing 
between the most southerly position that we can assign to the 
fleet of Agathocles, and the limit of the total phase. To 
obviate this discordance, it is only necessary to suppose an 
error of about three minutes of arc in the computed distance 
of the centres of Sun and Moon at conjunction, a very incon- 
siderable correction for a date anterior to the epoch of the 
tables by more than twenty-one centuries. 



ECLIPSES OF THE MOON, 103 

ECLIPSES OF THE MOON. 

In consequence of the ecliptic limits of the Sun exceeding 
those of the Moon, there are more eclipses of the former lumi- 
nary than of the latter, but owing to the comparatively small 
tract of the Earth's surface to which a solar eclipse is visible, 
the eclipses of the Moon are more frequently seen at any par- 
ticular place than those of the Sun. In point of interest and 
astronomical importance, however, they fall very far short. 

Eclipses of the Moon are either partial or total. The mag- 
nitude^ if partial, and the continuance of obscuration, if total, 
depend upon the direction of her transit through the Earth's 
shadow^, which is sufficiently broad at the distance of the Moon 
to allow of her being hidden by it one hour and fifty minutes, 
when a central passage through it takes place. 

The shadow of the Earth consists of a dark cone, sur- 
rounded by a lighter shade, termed the penumbra, which arises 
from a portion only of the Sun's rays being obscured. At that 
part of the conical shadow where the Moon traverses it, the 
diameter is between three and four times greater than her 
mean distance from the Earth, or, roughly speaking, 800,000 
miles. An eclipse may continue as long as five and a half 
hom's, reckoning from the first to last penumbral contact. 

It is not possible to ascertain, with any degree of accuracy, 
the time when the Moon first enters into the penumbra, for the 
darkening eflfect upon her disc is so slight that it requires some 
minutes to elapse before sufficient shade is produced to attract 
attention. Xeither does the moment of contact with the dark 
shadow^ admit of exact observation, and hence lunar eclipses are 
not of so much astronomical importance as eclipses of the Sun, 
as they afibrd but very imperfect determinations for differences 
of longitude. 

When the Moon is totally immei"sed in the shadow, she 



104 THE SOLAR SYSTEM. 

does not, except on some rare occasions, become invisible, but 
assumes a dul), reddish hue, somewhat of the color of tarnished 
copper. This arises from the refraction of the Sun's rays in 
passing through the Earth's atmosphere. During the eclipse 
of July 23, 1823, M. Gambart states he could distinctly see all 
the lunar spots, the general surface being of a deep red color. 
Ill that of September 11, 1802, Schroter remarked that the 
Earth's shadow was very brioht [seh?^ helle), and of a light gray 
color ; the penumbra w\^s not perceptible. Sir John Herschel 
observed the eclipse of December 26, 1833, during his voyage 
to the Cape of Good Hope, in latitude 26^ 30' south; the 
Moon continued conspicuously visible to the naked eye, while 
totally immersed in the Earth's shadow, and of a swarthy cop- 
per color, which changed to bluish green at the edges as the 
eclipse passed off. Sir J. Herschel thinks this remarkable phe- 
nomenon arose from the " accidental absence of clouds over a 
large portion of thatannulus of the Earth's atmosphere grazed 
by the Sun's rays at the time." The total lunar eclipse of 
March 8th, 1848, was characterized by similar phenomena. 
The spots upon the surface were distinctly seen by many ob- 
servers, even at the middle of the eclipse, and the general color 
of the Moon was a full, " glowing" red. Her appearance was 
so singular that many persons doubted her being eclipsed at all. 
Perhaps it may be worthy of remark, in connection with these 
facts, that the aurora horealls was seen in vivid streamers in 
various parts of England, Ireland, and the Continent. 

Instances of an analogous kind might be easily multiplied ; 
but the few we have mentioned will be sufficient to warn the 
reader of what he may expect to witness in a lunar eclipse. 

The earliest observations of lunar eclipses on record date in 
the year 719 and 720 e.g., reckoning according to the manner 
of astronomers, and w^ere taken at Babylon by the Chaldeans in 
the reign of Mardokempadius. Ptolemy has preserved these 



ECLIPSES OF THE MOON. 105 

ancient observations in his Almagest. The first of three edipses 
took place in the twenty-seventh year of the era of Xabonassar, 
the first of the reign of Mardokempadius, on the 29th of the 
Egyptian month Thoth, answering to the 19th of March, b.c. 
720, in onr mode of reckoning. The eclipse commenced about 
an hour after the rising of the moon, the greatest phase about 
2h. 30m. before midnight. It appeal's to have been total at 
Babylon. The second eclipse is dated in the twenty-eighth year 
of Xabonassar's era, on the night between the 18th and 19th 
of the month Tkoth, at midnight, or on the 8th of March, b.c. 
719 ; this was a partial eclipse. The third took place in the 
same year, on the loth of the month Phamenoth., correspond- 
ing to September 1st, b,c. 719 of our era. It began soon after 
the moon rose at Babylon, and continued three hours ; the mag- 
nitude was six digits on the northern limb, according to Ptolemy. 
Three other ancient eclipses fix the commencement of the im- 
portant era of Nabonassar in the year b.c. 746, and have like- 
wise indicated the existence of an acceleration in the mean mo- 
tion of the Moon, which was first detected by our countryman 
Dr. Halley. 

An eclipse in the nineteenth year of the Peleponnesian war, 
the fourth of the ninety-first Olympiad, produced very disas- 
trous consequences to the Athenian army, through the igno- 
rance of their general, Xicias. It is mentioned by Thucydides, 
Plutarch, and othei-s, and took place on the 57th of August, 
B.C. 412, being total at Syracuse, according to calculation upon 
modern tables. 

The lunar eclipse of March 1, 1504, proved of great service 
to Columbus, when his fleet was reduced to extremities for 
want of supplies. The islanders of Jamaica having refused to 
supply him, he threatened them with a deprivation of the 
Moon's light, as a punishment for their obstinacy. His threat 
was treated at fii^t with indifference ; but when the eclipse ac- 

5* 



106 '-^HE SOLAR SYSTEM. 

tually commenced, the barbarians vied with each other in the 
production of the necessary supplies for the Spanish fleet. 
Baron de Zach has satisfactorily shown that this echpse must 
have occurred on the above date. It was observed at Ulra, in 
Germany, by Stoffler, and at Nuremberg by Bernard Walther, 
and began about six o'clock in the evening, at Jamaica, which 
perfectly accords with the words of Columbus in describing the 
phenomenon. 



CHAPTEE YII. 

THE SUPERIOR PLANETS. 

MARS. $ 

ITfE now arrive at the planet Mars, which, in several respects, 
' ' has a closer analogy to our own globe than obtains in the 
other members of the solar system. He usually shines with a 
full, red or fiery hght, and, when in opposition and perihelion, 
is a very conspicuous object in the midnight sky. 

The mean distance of this planet from the Sun is 145,205,000 
miles, but in consequence of the eccentricity of its orbit, the dis- 
tance varies between 131,656,000 miles, when the longitude is 
about 332*4°, and 158,754,000 miles, when its heliocentric 
position is about 152-4°, so that the difference between theperi- 
hehon and aphelion distances amounts to 27,098,000 miles. 
The sidereal period of Mars is 686d. 23h. 30m. 41s., and the 
mean synodic period, or interval between two oppositions, is 
7Y9d. 23h., or rather more than two of our years. 

The apparent diameter of the planet varies considerably. 
About the time of conjunction it is little more than four seconds 
of arc, while in opposition and perihehon it may attain more than 
thirty seconds. The real diameter is probably not less than 
4500 miles. 

The phase of Mars does not undergo such great alteration 
as we have observed in the inferior planets. About the oppo- 
sition, he appeal's perfectly round, while, as he recedes from 
that point, his illuminated disc gradually diminishes until he ar- 



108 THE SOLAR SYSTEM. 

rives at quadrature, when the form resembles that of our Moon 
a day or two before the last quarter, so that the planet is gen- 
erally seen gibbous, but never less than a semicircle, which is a 
sufficient proof that, receiving its light from the Sun, the orbit 
must be exterior to our own. The phases of Mars were dis- 
covered by Galileo soon after the telescope was invented. 

When viewed under proper optical power, the surface of this 
planet presents outHnes of continents, and seas similar to those 
on the Earth, and usually, white spots are discernible near the 
poles, w^hich, from their alternate diminution and increase, as 
the Sun possesses greater or less power on the surface, are con- 
jectured to be masses of snow. The color of the continents is a 
dull red, that of the seas greenish, as, by contrast with the for- 
mer, they should be. It is this prevailing color of the land 
w^hich gives the planet that ruddy light by which it is at all 
times readily distinguished from the other members of the sys- 
tem, and from the fixed stars. By observing the spots upon the 
surface, the time of the axial rotation of Mars has been deter- 
mined. The elder Cassini made it 24h. 40ra., a very near ap- 
proximation to the truth. Sir W. Herschel paid great attention 
to his observations of this planet, and fixed the time of revolu- 
tion at 24h. 39m. 21*6'7s., mean solar time, remarking that he 
did not consider this determination liable to a greater error than 
2-34s. Professor Madler, however, from observations between 
1830 and 1834, infers the time of rotation to be 24h. 37m. 20s., 
or two minutes less than it is given by Sir W. Herschel. In 
1781-3, the latter astronomer measured the positions of the 
bright polar spots on Mars, with the view of ascertaining the 
obliquity of his ecliptic, or the angular inclination of his equator 
to the plane of the orbit. He found these spots were not ex- 
actly at the poles, but that the circles of their motions were 
situated at latitudes 15^ or 80^, the luminosity extending oc- 
casionally as far south as the sixty-fifth parallel on Mars. By a 



MARS. 109 

great number of observations, Sir W. Herschel found that tlie 
axis of Mars is inclined to his orbit at an angle of 61^ 18', or 
59^ 42' to our echptic, the north pole being directed to longitude 
347° 47'. The obhquity on the globe of Mars is therefore 28^ 
42', so that his seasons are possibly not very different from our 
own. 

A very extensive atmosphere has been thought to suiTound 
the planet ; and to this dense envelope has been attributed the 
ruddy appearance he presents to the naked eye ; but recent ob- 
servations, and particularly those of Sir James South, go far to 
disprove its existence. The attention of astronomers had been 
closely directed to this subject after the announcement of an 
observation by Cassini in 1672. He affirmed that in the month 
of October he saw the star called ip in Aquarius become so 
faint, when six minutes distant from the planet's disc, that it 
could not be discerned in a three-feet telescope. This star is 
of the fifth magnitude, and consequently visible to the naked 
eye. In alluding to this observation. Sir W. Herschel gave one 
of his own, w^hich tended to show that the atmosphere of the 
planet, though moderately dense, could not be so extensive as 
Cassini had inferred, since he could trace a very small star (of 
the thirteenth or fourteenth magnitude) within a very short 
distance from the planet's limb. 

Astronomers have not yet succeeded in discovering a satellite 
to this planet, although some of the most perfect telescopes have 
been brought to bear upon it. Being situated at a greater dis- 
tance from the Sun than our globe, we might imagine it would 
stand more in need of an attendant to illuminate its otherwise 
dark nights, and regulate the tides upon its oceans. If one 
exist it is possibly very small and close to the planet, which 
would account for its having so long escaped detection. 

In the absence of a satellite to afford us a more exact value, 
we can only be said to have approximated to the mass of the 



110 THE SOLAR SYSTEM. 

planet. The French calculator, Burckhardt, assigns 1-2680337, 
which is adopted by astronomers at the present day, and indi- 
cates that the mass of our neighbor is seven times less than that 
of the Earth. 

Dr. Maskelyne, formerly Astronomer Royal, examined this 
planet with the view of ascertaining whether there is any sensi- 
ble difference between the equatorial and polar diameter ; but 
could perceive none. Sir W. Hei-schel considered the ratio of 
the equatorial to the polar diameter as 16 to 15 ; but an exten- 
sive series of observations, recently taken with the best instru- 
ments to be found in observatories, gives the compression much 
less, or the ratio of the diameter as 51 to 50, w^hich is probably 
nearer the truth. It is only at the oppositions, or about once 
in tw^o years, that w^e see the disc of Mars fully illuminated, 
consequently, the proper times for determining the diflference of 
diameter, or for any observations upon the appearance of the 
surface, are not of very frequent occurrence. 

A method of determining the amount of the solar parallax 
from observations of Mars at places differing greatly in latitude 
has been put in practice by several astronomers ; though not 
very successfully. It is with a view to the application of this 
method that observers are furnished in the Nautical Almanac 
with a list of stars lying near the path of the planet about the 
times of opposition. The differences of declination between 
the stars and planet are to be repeatedly measured on the same 
night at the various stations, either with a micrometer or on 
the divided circle of an equatorial instrument. Suppose one 
of the observations in the northern and another in the southern 
hemisphere. The differences of declination measured at the 
two places and reduced accurately to the same moment after 
correction for refraction, will exhibit a discordance, which, sup- 
posing the observations exact, must be equal to the sum of the 
parallaxes of the planet at the two stations. Hence, knowing 



MARS. Ill 

the geocentric latitudes, we easily ascertain the amount of the 
horizontal parallax of Mars. But as we also know the relative 
distances of the Sun and planet from the Earth at the time, a 
very simple proportion gives us the value of the solar parallax. 
Cassini, from observations of a similar kind, found it lO'l''; 
La Caille, 10-2"; and Du Sejour, 9'4'7''. 

The most ancient observation of Mars that has come to our 
knowledge is one reported by Ptolemy in his Almagest, Book 
X., Chap. 9. It is dated in the fifty-second year after the death 
of Alexander the Great, and four hundred and seventy-sixth of 
Nabonassar's era, on the morning of the 21st of the month 
Athir, when the planet was above, but very near, the star ^ in 
Scorpio. The date answers to b.c. 272, January 17, at 18h. 
on the meridian of Alexandria. 

An occultation of the planet Jupiter by Mars is recorded, 
on the 9th of January, 1591. Such a phenomenon would be 
extremely interesting, if viewed with the powerful telescopes so 
common at the present day. 

The tables employed in predicting the geocentric places of 
Mars were published at Eisenberg in 1811 by the Baron Yon 
Lindenau, and are chiefly dependent upon observations taken 
at our Royal Observatory. They are capable of correction, 
though sufficiently exact for most practical purposes : the error 
in longitude in 1847 exceeded half a minute of arc. It is 
understood that new tables of the movements of this planet, 
founded on the Greenwich observations, are in course of prep- 
aration by the computers employed upon the American Nau- 
tical Almanac^ a work which has recently been commenced 
under the auspices of the Government of the United States. 



CHAPTER VIII. 

THE MINOR OR ULTRA-ZODIACAL PLANETS. 

BETWEEN the orbit of Mars and that of the next of the older 
planets (Jupiter), there occurs an interval of no less than 
350 millions of miles, in which no planet was known to exist 
before the commencement of the present century. Three hun- 
dred years ago Kepler had pointed out something hke a regular 
progression in the distances of the planets as far as Mars, which 
was broken in the case of Jupiter, and he is said to have sus- 
pected the existence of another planet in the great space sepa- 
rating these two bodies. The question attracted little further 
attention until Uranus was brought to light by Sir William 
Herschel in 1781, when several German astronomers revived 
the opinion held by Kepler, and, guided by an empirical law 
of distances published by Professor Bode of Berlin, even ap- 
proximated to the period of the supposed latent body. Accord- 
ing to this law, the distance of a planet is about double that of 
the interior one, and half that of the one immediately exterior 
to it, and, roughly speaking, this rate of progression of the 
planetary distances is found to hold good with these exceptions. 
Mars is situated at a distance of about twice that of the Earth, 
but very much less than half that of Jupiter ; and again, Jupi- 
ter is located at half the distance of the exterior planet Saturn, 
but considerably more than twice that of Mars. If, therefore, 
another planet existed between the orbits of Mars and Jupiter, 
the progression in Bode's law, instead of being interrupted at 



THE MINOR OR ULTRA-ZODIACAL PLANETS. 113 

this point, might perhaps be found to hold good as far as Ura- 
nus, and for this reason an association of astronomers was 
formed under the auspices of the Baron de Zach of Gotha, and 
a regular plan of search was devised, with a view to the dis- 
covery of the planet, which was put in practice about the end 
of the last century. The result of this systematic examination 
of the heavens will presently appear. 

CERES. 

Professor Guiseppe Piazzi, the celebrated Director of the 
Observatory at Palermo, repeatedly sought for a star numbered 
in "Wollaston's catalogue, Mayer 87, but, finding none in the 
position there assigned, he observed all the stars of similar 
brightness in the vicinity. On the 1st of January, 1801, or 
about the time when the search for the supposed latent body 
was begun, he determined the place of an object shining as a 
star of the eighth magnitude, not far from the position of the 
missing one. On the following night the place w^as sensibly 
altered, the instrument employed by Piazzi showing a retro- 
grade movement in right ascension with a northerly one in dec- 
lination. It does not appear that any notice of this discovery 
was given until the 24th, when lett^'s were despatched to sev- 
eral astronomers, in which Piazzi states that he had detected a 
co7net in 51^ 47' of right ascension and 16^ 8' north decHnation. 
Owing probably to delays of post, no observations were made 
at any observatory, except Piazzi's, before the conjunction of 
the stranger with the Sun ; but on the publication of the whole 
series of positions observed at Palermo, the eminent mathema- 
tician and astronomer Professor Gaussof Gottingen, undertook 
the determination of the orbit of Piazzi's star by methods 
which he had recently devised, and announced that it revolved 
round the Sun in about 1652 days, at a mean distance of 2-735, 
— that of the Earth being called 1. This distinnce agreeing so 



114 THE SOLAR SYSTEM. 

closely with that indicated on Bode's law for the planet supposed 
to exist between Mars and Jupiter, astronomers were very soon 
induced to regard Piazzi's " comet" as in reahty a new primary 
planet, fulfilling, in a remarkable manner, the conditions in re- 
spect to distance from the Sun, which had been found to hold 
good for the other members of the planetary system. 

Piazzi named his planet Ceres Ferdinandea^ in honor of his 
patron the King of IS'aples ; but the Ferdinandeo. has been 
dropped by common consent, and the planet is now known as 
Ceres,^ The very neat and significant symbol ? w^as adopted 
by astronomers, on the suggestion of the Baron de Zach. 

The detection of Ceres, on her reappearance after conjunc- 
tion with the Sun, was a matter involving some little difficulty, 
and one that occupied the attention of the principal observei's 
on the continent. Bode mapped down all the stars of his great 
catalogue lying near the expected path of the planet, according 
to the calculations of- Professor Gauss, and in the morning hours 
of October, November, and December, 1801, he sought for it 
with his 3-J feet telescope by Dollond. Dr. Olbers at Bremen, 
and Baron de Zach at Gotha, were similarly engaged. On the 
m'ght of December 7, a suspicious object was remarked by the 
latter astronomer near the computed place, but he was not able 

* Laplace, writing to Baron de Zach in 1802, states that he had 
mentioned the discovery of the Sicih'an astronomer to Bonaparte, 
" who, in the midst of his great occupations, took a lively interest in 
the progress of the sciences, and particularly of astronomy." Bona- 
parte thought Juno was a preferable name to Ceres, and Laplace says 
he held the same opinion, since it appears natural to place Juno near 
Jupiter. He adds that a Latin name was better than a Greek one, the 
German astronomers having already suggested Hera ("'Hpa), the Greek 
name of Juno, for Piazzi's planet. On this subject the discoverer ob- 
serves — " J' espere queles astronomes qui sont gens paisibles, ne con- 
senteront jamais a appeller leur divinites du nom d'une deese si inqui- 
ete, jalouse et vendicative comme Junon." 



THE MINOR OR ULTRA-ZODIACAL PLANETS. II5 

to decide whether he had really seen the planet until the 31st 
of that month, when he had the satisfaction of observing it again, 
and finding no star in the position noted on the 7th. Dr. Olbers 
found it on the 1st of Januaiy, 1802, and, on the 13th of 
February, it was seen at our Royal Observatory by Dr. Maske- 
lyne. The calculations of Professor Gauss mainly contributed 
to the re-discovery, and, in fact, he has been considered by some 
astronomers as the second discoverer of the planet, which they 
imagine would have been lost but for the manner in which the 
future course was predicted by the profound mathematician of 
Gottiugen. Professor Bode says a friend of his could discern 
the planet without a telescope in March, 1802, when its bright- 
ness was about equal to that of a star of the seventh magni- 
tude ; but generally Ceres is just beyond unassisted vision, and 
would be more properly termed an eighth magnitude. 

The minuteness of the planet has prevented any exact de- 
termination of the real diameter. Schroter thought it exceeded 
1600 miles, while Sir W. Herschel's measures gave only 163 
miles, and this last measure is certainly luuch nearer the truth 
than the former. Observers have remarked a haziness sur- 
rounding the planet, which is attributed to the density and ex- 
tent of its atmosphere. The light is very slightly tinged with 
red. The mean distance from the Sun in 1850, is 263,713,000 
miles, and the length of a sidereal revolution 4*6033 years. 

PALLAS. 

In order to find Ceres the more readily, Dr. Olbers exam- 
ined particularly the configurations of the small stars lying 
near her path. On the 28th of March, 1802, after observing 
the planet, he swept over the north wing of Yirgo with an in- 
strument termed a " Cometen Sucher," and was astonished to 
find a star of the seventh magnitude forming an equilateral 
triangle with 20 and 191 of Bode's Catalogue, where he was 



116 THE SOLAR SYSTEM. 

certain no star was visible in January and February preceding. 
In the course of less than three hours he found the right as- 
cension had diminished and the north dechnation increased. 
On the following evening, as soon as twilight permitted, he 
looked again for his star : it no longer formed an equilateral 
triangle with the stars above mentioned, but had moved con- 
siderably in the direction indicated by the previous night's ob- 
servations. On the 30th, after again observing the planet, Dr. 
Olbers wrote to Bode at Berlin, and to Baron de Zach, giving 
an account of his discovery. " What a singular accident," he 
exclaims, " was it by which I found this stranger in the same 
place, or only about twenty-six minutes (of space) north of the 
position where I had observed Ceres on the 1st of January." 
It was truly a fortunate circumstance. In the letter to Pro- 
fessor Bode, Dr. Olbers suggested Pallas as a name for a new 
member of the system. The elements of the orbit were quickly 
determined by Professor Gauss, who found the most remarka- 
ble peculiarity consisted in the great inclination of its plane to 
the ecliptic, owing to which the planet passed far beyond the 
limits of the ancient zodiac. The orbit was found to be an 
ellipse of not much greater eccentricity than that of Mercury, 
with a mean distance nearly the same as in the case of Ceres. Dr. 
Olbers pointed out that the orbits of the newly-discovered planets 
approached very near each other at the descending node of 
Pallas, a circumstance which induced his remarkable conjecture 
as to the common orio^in of these bodies. He thouQ^ht a much, 
larger planet had, in remote antiquity, existed near the mean 
distance of Ceres and Pallas, and that, by some tremendous 
catastrophe, this body had been shivered in pieces, — the two 
small planets being amongst the fragments. At the time this 
hypothesis was started it was certainly a bold one, but we shall 
presently see it is materially strengthened by the discoveries of 
later years. 



THE MINOR OR ULTRA-ZODIACAL PLANETS. 117 

When nearest to the Earth in opposition, Pallas shines as 
a full seventh magnitude, with a fine yellowish light, as we 
can testify from observations under very favorable circum- 
stances in March, 1848. Some astronomers have noticed a 
haziness round the planet, but not so strongly marked as with 
Ceres : this appearance is considered due to extensive atmos- 
phere. Sir W. Herschel thought the diameter of Pallas a little 
over seventy -five miles, while Schroter made it 770 at least. 
These discordances prove the great difficulty and uncertainty 
of such observations. Dr. Lamont, from two nights' measures 
with the powerful telescope at the Royal Observatory, Munich, 
found the diameter GYO miles, which is probably a fair approx- 
imation to the truth. • The mean distance of the planet from 
the Sun is 264,256,000 miles, and the time occupied in one 
sidereal revolution 4*61 To years, or 1687 days. 

JUNO. 

Professor Harding of Lilienthal occupied himself in the forma- 
tion of charts of small stars lying near the paths of Ceres and 
Pallas, with a view to assist the identification of these minute 
bodies. On the 1st of September, 1804, at ten o'clock in the 
evening, he noticed an object shining as a star of the eighth 
magnitude, near the stars 93 and 98 in Pisces, of Bode's great 
catalogue. The position was in right ascension 2^ 24' and 
north declination 0^ 37'. On the evening of the 4th he re- 
examined the neighborhood, and soon discovered that the star 
had altered its place. The right ascension had diminished, and 
the declination was now south. On the 5th and 6th he observed 
it more accurately, and finding that the positions deduced from 
the observations confirmed the retrograde motion indicated by 
the estimations on September 1st and 4th, he announced the 
discovery to Dr. Olbers, at Bremen, on the 7th, who saw it the 
same evening. Professor Harding named his planet Juno^ 



118 THE SOLAR SYSTEM. 

and chose for a symbol | , representing a sceptre crowned by a 
star. 

Juno usually appears like a star of the eighth magnitude, 
of a somewhat ruddy color. Her period of revolution round 
the Sun is 4*3594 yrs., and her synodic period about 474 days. 
The mean distance from the Sun is 254,312,000 miles. This 
planet was detected with a telescope of about thirty inches focal 
length, and two inches aperture, which would not show the belts 
of Jupiter, as the author has been assured by Sir James South, 
who saw the instrument at Gottingen. 

VESTA. 

Dr. Olbers, following up his idea respecting the origin of the 
zone of planets, considered, from the mutual intersection of the 
orbits of the three already found in Virgo and Cetus, and the 
explosion must have taken place in one or other of those re- 
gions, and consequently all fragments should pass through 
them. Provided with an ordinary night-glass, he examined 
every month the small stars in Yii-go and Cetus, w^hichever 
constellation w^as nearest the opposition. On the evening of 
the 29th March, 1807, soon after eight o'clock, while occupied 
in sweeping over the north wing of Virgo, as a part of his 
plan he discovered an object shining like a star of the sixth or 
seventh magnitude, west of Flamsteed's 20 Virginis, which he 
knew at once to be a planet, inasmuch as the previous exami- 
nation of the vicinity had indicated no star in the position of 
the strano'er. He satisfied himself that it was reallv in motion 
on the same evening, and continuing his observations until the 
2d of April, he obtained sufficient evidence to justify the public 
announcement of his discovery of another new planet. Accord- 
ingly, on the following day, he wrote to Professor Bode of 
Berlin, the editor of the Astronomische Jahi^huch^ and to Baron 
de Zach of Gotha, who conducted a most valuable periodical, 



THE MINOR OR ULTRA-ZODIACAL PLANETS, ng 

entitled Monatlieke Corresjyondenz. In his letter to Berlin, 
Dr. Olbers particularly mentions tliat his second discovery was 
not the result of accident, but of a systematic search for a body 
of this nature, guided by considerations already noticed. He 
adds, he should certainly have found the planet a fortnight ear- 
lier if moonlio;ht and unfavorable weather had not interfered 
with his observations, and remarks in conclusion, that neither 
Schroter and Bessel, with thirteen-feet and fifteen-feet reflecting 
telescopes, nor he himself, with an excellent achromatic by Dol- 
lond, could perceive any difference in appearance between this 
planet and a fixed star ; it shone with a somewhat reddish but 
very bright light, without planetary disc, or any surrounding 
nebulosity. At the request of Dr. Olbers, Professor Gauss 
undertook to name the planet, and decided upon Vesta^ a name 
highly approved of by the discoverer. The symbol chosen is 
g , to represent an altar with the sacred fire burning upon it. 
Professor Madler has carefully measured the diameter of Yesta 
with the famous telescope by Fraunhofer, erected at the Obser- 
vatory of Dorpat in Russia. A mean of several nights' measure 
makes the real diameter about 295 English miles. 

Vesta performs her revolution in 3'6284 yrs., at a mean 
distance of 225,000,000 of miles. Her orbit is httle more ec- 
centrical than that of Ceres. Xear her opposition to the Sun 
she appeal's the brightest of all the minor or ultra-zodiacal 
planets, and a person with good sight may often distinguish her 
without a telescope. The reddish tinge noticed by the astrono- 
mers at Lilienthal soon after the discovery of the planet is prob- 
ably due to some pecuHarity in the specula of their instruments ; 
but it is well known persons differ widely in their appreciation 
of coloi-s in the heavenly bodies. Some observers consider Yesta 
perfectly white ; while the author has repeatedly examined her 
under various powers, and always received the impression of a 
pale yellowish cast in her light. 



120 THE SOLAR SYSTEM. 

Dr. Olbers continued his systematic examinations of the 
small stars in Virgo and Cetus between the years 1808 and 
1816, and was so closely on the watch for any moving body, 
that he considered it very improbable a planet could have 
passed through either of these regions in the interval without 
being detected. Xo further discovery being made, the plan 
was relinquished in 1816. 

ASTR^A, 

After Dr. Olbers had discontinued his search for planets, 
the subject appears to have attracted little attention until M. 
Hencke, an amateur astronomer, at Driessen in Prussia, entered 
upon it with a zeal and diligence that could hardly fail in pro- 
ducing some important result. For fifteen years it is under- 
stood this gentleman had occupied himself in a strict survey of 
the zone of the heavens comprised within the charts published 
by the Royal Academy of Berlin. These charts contain all 
stars to the ninth magnitude inclusive, 15° on each side of the 
equator, and are complete for about two thirds of the hour of 
right ascension. M. Hencke extended these maps by the inser- 
tion of smaller stars, and his immediate object being the discov- 
ery of a new planet, he previously examined the configuration 
of the stars, so that by obtaining a close acquaintance with cer- 
tain parts of the heavens he could readily detect any moving 
body on its passage through them. The 8th of December, 
1845, was destined to be the epoch of M. Hencke's first discov- 
ery. While engaged on the evening of that day in comparing 
Professor Knorre's map (Hour iv. of right ascension) with the 
heavens, he noticed what appeared to be a star of the ninth 
magnitude, between two others of the same brightness in 
Taurus, w^hich had not been noted in his previous examina- 
tions. AYithout waiting for any further observations, M, Hencke 
wa-ote to Professors Encke and Schumacher, stating his reasons 



THE MINOR OR ULTRA- ZODIACAL PLANETS. 121 

for supposing that he had detected a new planet. On the 14th 
of December the astronomers of the Observatory at Berhn found 
the stranger in a position where no star appeared on Professor 
Knorre's excellent chart, and the motion was easily perceived 
the same evening. Information of the discovery reached this 
country in letters from Professor Schumacher on December 19, 
and the planet was observed on the 24th. M. Hencke having 
requested the celebrated Prussian astronomer Encke to name his 
new planet, the Professor fixed upon Astrcea, The period of 
revolution is found to be 1511 days, and the mean distance of 
the planet from the Sun 245,622,000 miles. 

Astraea will not be seen without a tolerably good telescope ; 
and, however pow^erful may be the instrumental means em- 
ployed, it is necessary to have a pretty exact knowledge of her 
position in respect to the neighboring stars, to guard against 
observing a wrong object. At the opposition in 1847 she was 
not brio'hter than a star of the tenth mao;nitude, and no charts 
of the heavens hitherto published contain stars of so faint a 
class. Two months afcer opposition she had diminished into a 
twelfth magnitude, and was therefore observable only in the 
most powerful telescopes. Under the most favorable circum- 
stances, or when the planet comes into opposition and perihelion 
at the same time, it will but little exceed in brightness a star 
of the ninth class. 

HEBE. 

Encouraged by his success, M. Hencke continued his search 
for planetary bodies, extending and verifying the Berlin Aca- 
demical charts, and by frequent comparison with the heavens 
acquiring an extensive knowledge of the configurations of the 
smaller stars in certain regions about the equator and ecliptic. 
On the evening of the 1st of July, 1847, he noticed an object 
shining as a star, a little less bright than the ninth magnitude, 



122 ^-^-S SOLAR SYSTEM. 

whicli was not marked on Dr. Bremiker's chart for the seven- 
teenth hour of right ascension, nor observed in M. Hencke's pre- 
vious search about the neighborhood. At raidnight, on July 
3, it had retrograded in right ascension, leaving no doubt of its 
planetary nature, and showing by the direction and amount of 
its motion that it formed another member of the ultra-zodiacal 
group. Information of the discovery was circulated by M. 
Hencke on the following day, and the planet was soon recog- 
nized at the principal observatories of Europe. The illustrious 
mathematician and astronomer, Professor Gauss, was deputed 
by the discoverer to select a name for the stranger, and it was 
soon known as the planet Hehe^ with a cup for the symbol, em- 
blematic of the office of the goddess in mythology. There is 
decidedly a ruddy tinge about this planet from which Astraea 
is free. The mean distance from the Sun is about 231,089,000 
miles, and the time occupied in one sidereal revolution 3*7'761 
years. The orbit is very eccentrical, and inclined more than 
14^ to the plane of the ecliptic. 

IRIS. 

The next two members of this remarkable gi'oup in order 
of discovery were found by the author at the observatory erected 
by Mr. Bishop in the grounds of his private residence in the 
Regent's Park, London. So early as April, 1845, a search for 
a planetary body was commenced, but in consequence of other 
classes of observation, then more particularly followed up, no 
extensive or systematic plan of examination of the heavens was 
attempted. ' In JSTovember, 1846, a rigorous search was under- 
taken, the Berlin Academical charts being employed as far as 
they extend, while ecliptical charts, including stars to the tenth 
mao'nitude inclusive, were formed for other parts of the heavens, 
where the echptic falls beyond the declination hmits (15'^ IST. to 
10° S.) of the Berlin maps. 



THE MINOR OR ULTRA-ZODIACAL PLANETS. 123 

On the evening of the 13th of August, 1847, after nine 
month's close observation on the above system, an object re- 
sembhng a star of the eighth magnitude was discovered in the 
immediate vicinity of 63 Sagittarii, which had not been noticed 
at any former time. Its planetary nature being immediately 
suspected it was attentively observed, and in less than half an 
hour the motion in right ascension was detected. In the course 
of an hour the planet had retrograded two seconds of time, a 
sufficient change of place to be indubitable. An announcement 
of the discovery was given to astronomers generally on the fol- 
lowing morning, and observations were soon obtained at most 
of the European observatories. The name fixed upon for this 
new member of the solar system is /m, which appears to have 
met with general approbation amongst astronomers. The sym- 
bol is due to Professor Schumacher, and is composed of a semi- 
circle representing the rainbow, with an interior star, and a base 
line for the horizon. As an attendant upon Juno, the name 
was not inappropriate at the time of discovery, when Juno was 
traversing the 18th hour of right ascension, and was followed 
by Iris in the 19th. 

Several observers have remarked decided variations in the 
light of this planet which are not accounted for by change of 
distance from the Earth and Sun, and which there is strono- 
reason to suppose, in a great measure, independent of atmos- 
pheric conditions. If Olber's hypothesis with regard to the 
origin of this zone of planets be correct, these variations may 
possibly be caused by axial rotation. 

The i^eriod of revolution of Iris is 3*6844 yrs., or 1346 
days, and the corresponding mean distance from the Sun 
227,334,000 miles. IsTo approximation to the diameter of the 
planet has yet been obtained. 



124 THE SOLAR SYSTEM. 

FLORA. 

Continuing the 23lan of observation already described, the 
author noticed at 11 p.m. on the 18th of October, 1847, an ob- 
ject resembhng a star of the eight or ninth magnitude, ^vhich 
had not been previously visible in the position it then occupied. 
Its right ascension was 5h. 3m. 39s., and its north declination 
14° 4', it was therefore situate in the constellation Orion, or on 
the borders of Orion and Taurus. Clouds covered the heavens 
soon afterwards, and precluded further observation until about 
3 A.M. on the 19th, when the micrometer speedily revealed a 
direct motion in right ascension of about two seconds of time 
in the four hours elapsed since the discovery, and the declina- 
tion had also changed a little, the object having slightly ap- 
proached the equator. The alteration of position was quite 
large enough to prove the nature of the stranger, and it was 
announced to astronomei's on the same morning as the ninth 
member of the group of small planets, not far from its station- 
ary point. At the suggestion of Sir John Herschel, the new- 
planet received the name Flora^ and a flower, the *'Rose of 
England," was chosen as the symbol. The period of revolution 
is shorter than that of any other of her companion-planets, 
being about 1193 days only. Flora, therefore, comes after 
Mars in order of mean distance from the Sun, and approaches 
nearer to the Earth than the rest of the group to which she 
belongs. The semi-axis major of the orbit, or mean distance, is 
209,826,000 miles. The planet is somewhat ruddy, but with- 
out any hazy appearance, such as might be supposed to arise 
from an extensive atmosphere. On more than one occasion, 
when viewing it under high magnifying powers, the author 
has fancied he could perceive a measurable disc, but cannot 
place imphcit con"Sdeuce in the observation. At the time of 
the opposition in 1847, thehght of the planet was equal to that 



THE 21 IX OR OR ULTRA-ZODIACAL FLAXETS. 125 

of a star of the eightli magnitude. Flora and Iris were dis- 
covered with an eqiiatorially-mounted actiromatic telescope, 
having an object glass of seven inches aperture, and about 
eleven feet focal length ; the power employed being about 45. 

METIS. 

In the year 1848 another member of this interesting group 
was brought to light by Mr. Graham at the private observatory 
of Markree Castle, Ireland, under the direction of Mr. Cooper. 
Having formed a chart of the stars near the equator in the 
14th hour of right ascension, on a more extendt^d scale than 
that of the Berlin charts, he remarked on the 25th of April a 
star of the tenth magnitude in a position where none had been 
visible before, and noted it down for re-examination. On the 
following evening this object was found to have retrograded 
one minute, thus leaving no doubt of its planetary nature. 
On the 27th the discovery was announced to several astrono- 
mers in this country and on the Continent, and speedily be- 
came generally known through the circulars issued by Professor 
Schumacher. The position of the 2:)lanet at the time it was 
fii-st detected, was in right ascension 14h. 56m. 38s., and south 
declination 12^ 35', or in the zodiacal constellation Libra. 

The name selected for this planet is Metis, with an eye and 
star for a symbol, 

We are indebted to Mr. Graham, the discoverer, and to 
Dr. Luther, of Berhn, for our knowledge of the form of the 
orbit. Their calculations assign a periodic time of 1347 days, 
or 3*686 years, the corresponding mean distance from the Sun 
being 227,387,000 miles. 

The planet is fainter than either of the two discovered in 
England in the previous year, and a good telescope will be re- 
quired to show it well. 



126 THE SOLAR SYSTEM. 



HYGEIA. 



On the 12tli of April, 1849, Dr. Annibal de Gasparis, as- 
sistant astronomer at the Royal Observatory at Naples, while 
comparing Steinheil's chart for hour xii. of right ascension with 
the heavens, perceived a star of between the ninth and tenth 
magnitude in a position which he had found vacant at previous 
examinations of this region. Unfavorable weather prevented 
his observing it on that evening, but on the 14th he ascertained 
that it had sensibly changed its place, and was, therefore, a new 
planet, the amount of its motion showing that it must belong 
to the group of small planets between Mars and Jupiter. The 
position on the 14th was in A. R. 182^ 59' X. P. D. 97^ 28'. 
The discovery was announced to astronomers generally by 
M. Fabri Scarpellini, Secretary of the Corresioondenza Scien- 
tifica at Rome, and Professor Schumacher as usual issued his 
printed circular from Altona on the 11th of May. Professor 
Capocci, Director of the Neapolitan Observatory, named the 
new planet " Igea Borbonica ;" but it is universally termed 
Hygeia^ the unnecessary appendage " Borbonica" being drop- 
ped, as was the case with the complimentary additions to the 
names of the planets of Piazzi and Olbers. 

The elements of Hygeia are not yet very exactly known. 
We are certain, however, that the mean distance is greater than 
in the orbit of any other member of this group, the best cal- 
culations making it about 300,322,000 miles, corresponding to 
a revolution in 5*594 yrs., or 2044 days. Between the mean 
distances of Flora and Hygeia, those of all the other small 
planets are included. 

At no time since the discovery has Hygeia equalled in 
brightness an ordinary ninth magnitude, consequently good 
telescopes are required to observe her w^ell. We know nothing 
at present respecting her diameter. 



THE MINOR OR ULTRA-ZODIACAL PLANETS. 127 



PARTHEXOPE. 

On the occasion of the discovery of Hygeia, it appears Sir 
John Herschel had suggested that Parthenope ^You]d be a very 
appropriate name as memoriahzing the site of the discovery ; 
the nymph having given her name to the city now called 
Naples. Signor de Gasparis states that he used his utmost 
exertions to realize for Sir John Herschel a Parthenope in the 
heavens, and his endeavors were crowned with success on the 
11th of May, 1850. On the evening of that day he found an 
object shining as a star of the ninth magnitude in A. R. 230"^ 
22', and N. P. D. 100^ 35', which he soon ascertained to be 
a new planet from its motion in right ascension. He gave im- 
mediate notice of his discovery, and before the end of the 
month the planet was observed at many of the European ob- 
servatories. 

The elements of Parthenope have been calculated by several 
astronomers, but it is not to be expected that we can know 
them with any degree of accuracy in so short a time after the 
first detection of the planet. The latest results indicate a mean 
distance of 233,611,000 miles, and a corresponding period of 
1401 days, or 3-838 years. 

VICTORIA. 

On the evening of the 13th of September, 1850, the au- 
thor noticed a star of the eighth magnitude in the constellation 
Pegasus, near another smaller one frequently examined on 
predous occasions, without any mention being made of its 
bright neighbor. Its peculiar bluish. light satisfied him at once 
as to its planetary nature, and the micrometer was introduced 
to ascertain the difference of right ascension between the two 
objects, and to obtain satisfactory proof of the discovery of a 
new planet, — for the eleven known members of the extra- 



128 THE SOLAR SYSTEM. 

zodiacal group were all in different positions, according to cal- 
culation. In less than an hour the brighter star had moved 
westward about two seconds of time, so that no doubt could 
be entertained in respect to its nature and position in the Solar 
System ; this amount of retrograde motion in an hour being 
such as a planet of the group between Mars and Jupiter would 
exhibit in the direction of the stranger. 

The name selected for the twelfth member is Victoria., 
which we think is perfectly consistent with conventional usage 
amongst astronomers in reference to small planets ; the rule 
hitherto followed requiring a female name, taken either from 
the Greek or Roman Mythologies. The name has been readily 
accepted, as far as we are aware, by all the principal astrono- 
mers of Europe. The symbol is a star surmoimted by a laurel 
branch. 

The period of Victoria is 1302 days, and her mean dis- 
tance from the Sun 222,373,000 miles, which places her be- 
tween Flora and Yesta. The orbit is more eccentrical than 
that of Flora, though less so than that of Iris. At her maxi- 
mum brilliancy she will be seen with very small optical power, 
resembling a bluish star of the eighth magnitude; at other 
times, when her distance from us is much greater, the light 
will hardly exceed that of a star of the eleventh class. 

EGERIA. 

The discovery of Victoria (which afforded the first instance 
on record of the detection of three planets by the same ob- 
server), was quickly followed by that of another small j^lauetary 
body by Dr. Annibal de Gasparis, at the Royal Observatory, 
IS'aples. In this case, a star map was not the means of bring- 
inor to lio-ht the little wanderer, but its existence was indicated 
by a series of observations in zones of declination, which the 
able and energetic astronomer had instituted for the express 



THE MINOR OR ULTRA-ZODIACAL PLANETS. 129 

' purpose of finding new planets. On the 2d of November, 
1850, Dr. Gasparis met with the thirteenth member of the 
extra-zodiacal group, in the constellation Cetiis, or in that 
region of the heavens which Olbers had considered the most 
convenient for his periodical examinations, since the nodes of 
Ceres, Pallas, <fcc., appeared to lie in that direction. This 
planet was much fainter than Victoria, and probably, in its 
most favorable position in respect to the earth, it will not excel 
in intensity of light a star of the ninth magnitude. 

M. Le Yerrier, having been deputed by the discoverer to 
name his prize, has proposed Egeria^ the councillor of ISTuma 
Pompilius. The period of a sidereal revolution is 1505 days, 
and the mean distance from the Sun 244,940,000 miles. The 
orbit is more inclined to the plane of the ecliptic than that of 
any other planet, Pallas alone excepted ; its eccentricity is very 
nearly the same as in the orbit of Yesta. 

IRENE. 

The next member of the group of small planets in the 
order of discovery was found by the author in the constellation 
Scorpio, on the 19th of May, 1851, and four days later by 
Dr. Gasparis, at IS'aples. It appeared like a star of between 
the eighth and ninth magnitudes, with a full blue light, and 
seemed to be surrounded by a faint nebulous envelope or at- 
mosphere, which could not be perceived about stars of similar 
brightness. The nature of this object was satisfactorily estab- 
lished within half an hour from the first glimpse of it on tlie 
19th of May ; repeated examinations of the vicinity on previous 
occasions, having indicated no star in the position of the 
stranger. At the recommendation of Sir John Herschel the 
new planet was named Irene^ in allusion to the peace prevail- 
ing at the time in Europe ; the symbol proposed being a dove 
with an olive branch and star on head. 

6* 



130 '^HE SOLAR SYSTEM. 

The discovery of this planet is too recent to allow of any 
exact knowledge of its path in space, but the most trustworthy 
calculations hitherto published assign it a mean distance of 
246,070,000 miles, and a corresponding period of revolution of 
4*15 years. 

EUNOMIA. 

On the night of July 29, 1851, another small j^lanet was 
discovered by Dr. Gasparis at Naples, in hour xviii. of right as- 
cension, a little below the ecliptic. It shone as a fine star of 
the ninth magnitude ; but owing to its low situation in the 
heavens, was not so generally observed during its first appari- 
tion as some of the other newly-found bodies. Dr. Gasparis 
named his planet Eunomia^ who, in classical mythology, was 
one of the Seasons, a sister of Irene. 

The period of Eunomia is 1574 days, or 4*308 years, and 
her mean distance from the Sun is about 252,300,000. 

We have already alluded to the near approximation of the 
orbits of the small planets at the points of mutual intersection, 
a circumstance which induced Olbers and many other astrono- 
mers to consider these bodies as the fragments of a large planet 
formerly revolving at about the same mean distance from the 
Sun, which had been shivered into pieces by some great inter- 
nal explosion or an external shock. The idea of the German 
astronomer has been so strongly countenanced by the discov- 
eries of the last five years, that we cannot fairly reject it until 
another theory has been advanced which would account equally 
well for the peculiarities observed in this zone of planets, how- 
ever unwilling we may be to admit the possibility of such tre- 
mendous catastrophes, and notwithstanding the great difference 
in the mean distances of Flora and Hygeia, the innermost and 
outermost of the zone. Yet it is singular that this group ap- 
pears to separate the planets of small mass from the greater 
bodies of the system, the planets which rotate on their axes in 



THE MINOR OR ULTRA-ZODIACAL PLANETS. 13] 

about the same time as the Earth, from those which are whirled 
round in less than half that interval, though of ten times the 
diameter of our globe ; and it may yet be found that these 
small bodies, so far from being portions of the wreck of a great 
planet, were created in their present state for some wise pur- 
pose, w^nch the progress of astronomy in future ages may even- 
tually unfold. 



CHAPTER IX. 

JUPITER. U 

JUPITER is the next planet in order of distance from the 
Sun and the largest in the system, presenting a bulk more 
than twelve hundred times greater than that of the Earth. He 
revolves round the Sua in an orbit but shghtly inclined to the 
plane of the echptic, at a mean distance of 495,817,000 miles, 
in a period of 4332 days, or a little less than twelve of our 
years. Owing to the eccentricity of the orbit, the planet is 
nearer the Sun by 47,760,000 miles when its heliocentric lon- 
gitude is about 11^ than w^hen it is situated in the opposite 
point of the echptic, or the radius-vector varies in length be- 
tween 471,937,000 miles and 519,697,000 miles. The appa- 
rent diameter of Jupiter is about 47'' at the oppositions, and 
30'' or rather more, near the times of conjunction. The real 
mean diameter is 88,780 miles, according to the observations 
of Professor Struve, but the difference between the polar and 
equatorial diameters is considerable, and will strike the eye the 
moment the planet is seen in a telescope under proper magnify- 
ing power. The most recent measures make the ratio of the 
polar to the equatorial diameter as 947 to 1000. Professor 
Struve considered it much more unequal, and his numbers would 
assign 92,130 for the equatorial, and 85,430 for the polar diam- 
eter. The mean circumference of the planet exceeds 2,789,000 
miles. 

On examining the surface of Jupiter with telescopes, "we see 



J V PITER. 133 

no appearance of regular continents or seas as on the surface 
of Mars ; but dark streaks, or, as they are termed, helts^ are 
found to cross his disc, presenting similar forms to some of the 
modifications of cloud in our own atmosphere. Occasionally 
these belts retain nearly the same form and positions for months 
together, while at other times they undergo great and sudden 
changes, and, in one or two instances, have been observed to 
break up and spread themselves over the whole disc of the 
planet. Generally there are two belts much more strongly 
marked than the rest, and retaining a higher degree of perma- 
nence, one situated a little north and the other a little south of 
the planet's equator. The prevailing opinion amongst astrono- 
mers in reference to the nature of these phenomena is, that they 
are produced by disturbances in the planet's atmosphere, which 
occasionally render its dark body visible, and, as the belts are 
found to traverse the disc in hues uniformly parallel to Jupi- 
ter's equator, we are naturally led to the conclusion that these 
disturbances are connected with the rotation of the planet upon 
its axis, which, as we shall presently see, is performed with 
wonderful rapidity. The belts were first observed by Zuppi 
and Fontana, at Xaples, soon after the invention of the teles- 
cope. 

In July, 1665, Cassini of Paris remarked a black spot of 
considerable apparent magnitude on the upper edge of the 
southern belt of Jupiter, which remained visible two yeai-s. 
This spot, or one supposed to be identical with it, has repeat- 
edly appeared since the time of Cassini, but at veiy irregular 
intervals. On the 11th of December, 1834, a remarkable spot 
was ' discovered on the northern belt by the observei-s at Cam- 
bridge. It was black and well defined ; about two thirds of 
its breadth was above the belt, and one third upon it. On the 
13th of the same month two spots were distinctly visible on 
the north belt, both well defined, but the following or eastern 



134 THE SOLAR SYSTEM. 

one (that of the 11th) being the larger. For some time these 
spots remained wholly unchanged, but, in 1835, they gradually 
faded away ; the principal one was seen at Cambridge for the 
last time on March 19th. Cassini noticed that the spot in 
July, 1665, appeared to traverse the disc from east to west : it 
was very conspicuous near the centre of Jupiter, and gradually 
faded away as it approached the w^estern limb : the motion 
seemed quickest when the spot was near the centre, and be- 
came slower towards the edge of the planet. Hence he inferred 
that it adhered to the surface and was carried across the disc by 
the rotation of Jupiter upon his axis, an hypothesis which would 
account fully for the appearances observed. By closely watch- 
ing the movements of the spot, the same astronomer ascertained 
that the time of revolution of the planet was about 9h. 56m. 
The spots of 1834-5, were carefully observed at Cambridge, 
and by the German astronomers M.M. Beer and Madler. Mr. 
Airy, on discussing the observations taken at Cambridge, de- 
duces 9h. 55m. 21 '33. for the time of diurnal rotation, and M. 
Madler 9h. 55m. 29*9s. This enormous globe, exceeding in 
diameter that of the earth eleven times, is, therefore, whirled 
round upon its axis in less than ten of our mean solar hours. 
A particle at the equator of Jupiter must consequently move 
with a velocity of more than 450 miles per minute, and it is 
easy to conceive how^ materially this rapid rotation would con- 
tribute to the generation of heat upon the surface of the planet, 
and in other ways tend to compensate for the effects we might 
expect to follow from the great distance of Jupiter from the 
Sun, which, without some alleviating agency, w^ould appear to 
render the planet an unfit abode for sentient beings. But it is 
further worthy of remark that the axis of rotation is very slightly 
inchned to the plane of the orbit, a circumstance which would 
produce one constant climate at any particular spot upon the 
surface of this immense globe, the regions in the immediate 



JUPITER. 135 

neighborhood of the poles alone excepted. The inclination of 
Jupiter's equator to his echptic, i.e.^ to the plane of his orbit, is 
3° 4' b"^ and the ascending node of the equator in 1850, is in 
longitude 314^ 45'. 

Jupiter is attended by four satellites or moons, Tvhich were 
discovered by Gahleo, at Padua, on the 8th of January, 1610. 
There has been a good deal of disputation concerning the 
claims of other astronomers to the honor of having first ob- 
served these interesting objects : our countryman, Harriot, was 
held to be the discoverer by Baron de Zach, who was certainly 
mistaken, as the facts brought to light respecting Harriot, by 
Professor Eigaud, have undoubtedly proved. Simon Marius 
asserted positively, in his Mundus Jovialis^ that he had re- 
marked the satellites on the 29th of December, 1609, at Ans- 
bach, in Bavaria, a point which w^as warmly disputed by Gali- 
leo, who termed Marius the " usurper of the system of Juf)iter." 
M. Delambre considered that the Mundus Jovialis^ so far from 
establishing the claim of Marius to the discovery, bore internal 
evidence to the contrary. At this distance of time it is safest, 
in forming an opinion, to be guided by the publications of each 
astronomer, and it is quite certain that the announcement of 
the existence of four Jovian satellites, by Galileo, in his I^un- 
cius Siderius^ preceded the first notification by Marius nearly 
two years. The earhest observations of Harriot date October, 
1610, or nine months later than those of Galileo. It is quite 
possible that the Italian and German astronomers may have 
discovered the satellites about the same time and entirely inde- 
pendently of each other, but the general opinion at the present 
day is in favor of priority on the part of Galileo. 

The newly discovered secondary planets were named Sidey^a 
Cosmica, or Medicea, by Galileo, in honor of his patron, Cosmo 
de Medici, and to distinguish them from one another, he pro- 
posed to give them the family names of the ruling house at 



136 'J^HE SOLAR SYSTEM. 

Florence. Simon Marius, on the contrary, suggested mytlio- 
logical names — lo, Europa, Ganymede, Callisto. Modern as- 
tronomers, however, have not thought it necessary to resort to 
any such special nomenclature, but identify the satellites as the 
first, second, third, and fourth, in order of their distance from 
the primary. 

The first sateUite presents an apparent diameter of 1 '015'' 
at the mean distance of Jupiter from the Sun, whence we find 
the real diameter to be 2440 miles. Its average distance from 
the centre of the planet is 278,500 miles, and the period of a 
sidereal revolution is Id. 18h. 27m. 33'50s. 

The second satellite appears under a diameter of 0'911'', or 
its real diameter is 2190 miles ; it is the smallest of the four, 
according to the observations of the eminent Russian astrono- 
mer. Professor Struve. The mean distance from the planet is 
443,300 miles, and the time occupied in one sidereal revolution 
is 3d. 13h. 14m. 36-4s. 

The third satelHte is seen under an angle of 1*488^' at the 
mean distance of Jupiter, whence its true diameter is 3580 
miles, considerably greater than that of the planet Mercury, It 
is the largest of the satellites, as the measures of Schroter and 
Struve have shown. The distance from the centre of the pri- 
mary is 707,000 miles, and the time of a sidereal revolution 
7d. 3h. 42m. 33-4s. 

The apparent diameter of the fourth satelHte is 1*273'', and 
its real diameter 3060. The distance from the planet amounts 
to 1,243,500 miles, corresponding to a sidereal revolution of 
16d. 16h. 31m. 49-7s. 

The satelHtes shine with the brilliancy of stars of between 
the sixth and seventh magnitude ; but owing to their proximity 
to the planet, which overpowers their light, they are invisible 
to the naked eye. There are some few instances on record where 
persons possessed of extremely good vision have fancied they 



JUPITER. 137 

could perceive one of these little moons without optical aid, but 
on applying the telescope, it has been found that three of the 
satelHtes have approached so near together as to appear like 
one — just perceptible to the unassisted eye. A very small op- 
tical power suffices to exhibit the satellites clearly as stars, but 
to see them with measurable discs requires the very best instru- 
ments yet constructed, and high magnifyers. 

If we adopt the values of the diameters resulting from the 
observations of Professor Struve, we shall find that, as viewed 
from the equator of Jupiter, the first satellite would appear by 
far the largest, its apparent diameter being greater than that of 
our Moon, or about 36 \ The second and third would seem to 
be 19' in diameter, wMe the fourth would subtend an ano-leof 
only 9', or about one quarter of the apparent diameter of the 
first. The second and third satelhtes might, therefore, be to- 
tally eclipsed by the fii*st, and the fourth by all the others. 
The diameter of the Sun, as seen from Jupiter, in perihelion, 
is 6-|- minutes of arc, so that total eclipses or occultations of the 
Sun are of common occurrence to the equatorial inhabitants of 
the planet, some portion of the sm-face necessarily suffering an 
entire deprivation of the solar light every time the first, second, 
or third satellite passes through the interior part of its orbit. 

The configurations of the satellites of Jupiter are continu- 
ally varying : sometimes their attendants ar-e all situated on 
one side of the planet, though more frequently one, at least, is 
to be found in each direction. Some few instances are on rec- 
ord when all four have been invisible for a short time ; such 
was the case on 2d Xovember, 1681, old style, according to 
an observation by Molyneux, and the same phenomenon has 
been witnessed on more than one occasion dining the present 
century. It is not so rare an occurrence to find only one satel- 
hte visible. The amateur astronomer will find thB tables of 
configuration of the satelhtes, given in our Nautical Almanac 



138 THE SOLAR SYSTEM, 

of great service to him in fixing upon the proper times for 
viewing these little moons in their most interesting positions 
relative to the primary. 

Sir William Herschel, by a long series of observations upon 
the satellites, inferred that they rotate upon their axes in the 
time of one synodical revolution round Jupiter, thus presenting 
an analogy to our own satellite. He was led to this conclusion 
on remarking the great changes in the relative brightness of 
the satellites in diflferent positions, which were found to follow 
such a lav*^ as was reconcilable only with this hypothesis. The 
orbits of the satelHtes, as seen from the Earth, are projected 
into very eccentrical ellipses in most situations ; but when our 
globe is in the line of nodes of any satellite its apparent path 
is a straight line. The real paths of the first and second sat- 
ellites do not difier sensibly from circles : those of the third and 
fourth are very slightly elliptical. The line of apsides of the 
third revolves in about 137 years, and that of the fourth in 
about 516 years. The line of nodes of the three exterior sat- 
ellites revolve in a retrograde direction, as is the case with the 
nodes of the lunar orbit; the period for the second is 30 years, 
for the third 140, and for iXiQ fourth 520 years. 

The eclipses and occultations of the satellites, and transits 
of the satellites and their shadows, over the disc of Jupiter, 
have long attracted considerable attention, as well on account 
of the interest attaching to their phenomena, as for their utility 
in practical astronomy. 

The shadow of the planet extends into space through a dis- 
tance of more than half the interval which separates the Sun 
from the Earth ; and, in consequence of the smallness of the orbi- 
tal inclinations of the satellites, the first, second,\i\di third 
suffer an eclipse in every revolution round the primary ; the 
fourth sometimes escapes altogether, or may suffer a partial 
eclipse only, a circumstance arising from the plane of its orbit 



JUPITER. 139 

being rather more inclined than with the others. The first en- 
trance of a satelHte into the shadow of Jupiter is called the 
immersion ; when it is just clear of the shadow, the emersion 
is said to take place. Soon after the conjunction of the planet 
with the Sun, the shadow is projected on its western side, and 
at this time, both the immersions and emersions of the third 
and fourth satellites may be observed, and occasionally those 
of the second^ but the emersions only of the first are visible, 
since it is so near the planet as to enter into the shadow behind 
the disc. About the oppositions, the immersions and emer- 
sions take place very near the limb of Jupiter : as he moves 
onward towards the Sun's place, the shadow is projected on his 
eastern side ; the immersions only of the first satellite are then 
visible, because on leaving the shadow it is occulted by the 
planet, while the immersions and emersions of the third and 
fourth satellites, and, more rarely, those of the second^ may be 
observed to the east of the planet. 

One of the most important results to which the observa- 
tions of the eclipses of Jupiter's satellites have conduced, is the 
detection of the measurable velocity of light, due to Olaus Eo- 
mer, a Danish optician and astronomer, about the year 1675. 
It had been remarked that the calculations always gave the 
times of the eclipses Avith errors of contrary signs when Jupiter 
was nearest to, and farthest from, the Earth. In the former 
case the eclipse occurred before the computed moment ; in the 
latter, the predicted time w^as invariably too early. These cir- 
cumstances led Romer to suspect that the anomaly arose from 
the hght having a greater distance to travel when Jupiter was 
in apogee than when he was nearest to the Earth, or about the 
times of opposition, and, on comparing the observations to- 
gether, it was inferred that light actually travelled at the rate 
of nearly 200,000 miles in a second, or that it would requiie 
16m. 36s. to traverse the diameter of the Earth's orbit. The 



140 THE SOLAR SYSTEM. 

corrections applied to the observed times on this hypothesis 
rendered them perfectlj accordant with one general theory of 
the satellites' movements, which before had failed to prove sat- 
isfactory. The eclipses are highly useful, also, for determining 
approximate differences of longitude between distant stations 
on the Earth's surface. We say approximate^ because there 
are difficulties in the way of their exact observation, which pre- 
clude the possibility of obtaining very accurate results from them. 

The theory of the motions of Jupiter's satellites has been 
fully developed by the celebrated mathematician, Laplace, and 
was published in his Mecanique Celeste. It is beyond the plan 
of this work to enter into any details on so comphcated a sub- 
ject; but there are one or two points in connection with the 
relative movements of the satelhtes which deserve especial no- 
tice. It appears that this singular relation subsists between 
the mean motions of the three interior ones — the mean angular 
velocity of the first satellite added to twice that of the third 
gives a sum exactly equal to three times that of the second, 
wherefore, if three times the mean longitude of the second 
satellite be subtracted from the mean longitude of the fii'st, 
plus twice that of the third, the remainder will be always con- 
stant, or as we find from observation, 180^. This curious ar- 
rangement has a most important effect in the system of the 
planet. It is thus seen, that for a long period to come, if the 
first and third satellites present their unilluminated sides to the 
planet, or are undergoing eclipse simultaneously, the second must 
be so situated as to afford considerable light to its inhabitants, and 
the same will occur in regard to the third satelhte, if the first and 
second are in such positions as to reflect no hght on the planet. 

The occultations of the satellites, which take place when 
they pass behind the disc of the planet, generally require much 
more powerful instruments for their satisfactory observation 
than the eclipses. With a telescope of adequate power, we 



JUPITER. 141 

may trace the gradual disappearance of the satelhte, from the 
first contact with the hmb of the planet to its final obscuration 
behind the disc, and, as viewed with such an instrument, these 
phenomena are highly interesting. The occultations of the 
fourth satellite are usually visible throughout, i. e, from disap- 
pearance to re-appearance ; those of the third also are frequently 
observable ; but it happens much more rarely that the com- 
plete phenomenon can be observed in regard to the second satel- 
hte, while the immersion and emersion of the fii^st can only be 
visible a day or two before or after the opposition of Jupiter, as 
at all other times either the immersion or emersion must hap- 
pen while the satelhte is obscured in the planet's shadow. Thus 
it most usually occurs, that from conjunction to opposition, the 
re-ap2oearances only of the first and second satellite can be observ- 
ed, and the disappearances only from opposition to conjunction. 
Perhaps the most interesting of all the phenomena of the 
Jovian system, are the transits of the satellites and their shadows 
over the dis-c of the planet, which occur when they are moving 
from east to west. With powerful telescopes the satellites are 
seen projected upon the disc, sometimes as lucid spots sensibly 
brighter than the general surface of the primary ;, while, on 
other occasions, they have been observed as dark spots, — which 
can only be accounted for by admitting that such spots really 
exist on the satellite itself, since the illuminated part of their 
disc must be turned towards the Earth at these times. Schroter 
and Harding noticed these varied appearances repeatedly ; and, 
so long ago as the middle of the seventeenth century, Cassini 
had discovered that the satellites were sometimes visible during 
their passage across Juj)iter's disc, though he could perceive no 
trace of them on other occasions. Professor Bond, of Cam- 
bridge, U. S., has detailed some curious observations made by 
him with the great telescope under his direction, on the transit 
of the third satellite and its shadow. On the 28th of January, 



142 THE SOLAR SYSTEM. 

1848, it was seen as a black spot, well defined, and again on 
the 11th of March. On the 18th of the latter raonth it en- 
tered upon the disc as a very bright spot, more biilliant than 
the surrounding surface : twenty minutes later it had decreased 
in brightness, so as to be hardly perceptible, and in a few 
minutes a dark spot appeared suddenly in its place, and was 
seen nearly two hours and a half. It was conspicuous enough 
to be easily measured with a micrometer, being perfectly black 
and nearly round. This spot is stated to have been observed 
on the satelhte. The same astronomer has also seen the first 
and fourth satellites like dusky spots projected on the disc of 
Jupiter, and the same appearance has been repeatedly noticed 
in this country during the transits of the fourth satelhte. The 
shadows are uniformly black and larger than the satelhtes 
themselves : they are consequently more readily perceived, 
though powerful instruments are required for the proper obser- 
vation of either satelhtes or shadows at these times. Before 
the opposition of Jupiter to the Sun the shadov^^ precedes the 
satellite, hut foUoivs it after the opposition. 

The mass of Jupiter is much greater than that of any other 
planet, and an accurate knowledge of it is therefore of consid- 
erable importance in astronomy. We cannot predict the true 
positions of the small planets without taking into account the 
influence of Jupiter upon their movements, and this action is 
very sensible for some of the larger bodies of the system. 
Newton, Lagrange, and Laplace, considered the mass of the 
Sun to be to that of the planet nearly as 1067 to 1. Bouvard, 
in his Tables of Jupiter, assumes it as 1070 to 1. The calcu- 
lations of Professor Encke in 1826, relative to the disturbances 
produced by the planet on the elements of Yesta, indicated the 
necessity of a very sensible increase on the received mass, which 
appeared to be more nearly as 1050 to 1, a result confirmed in 
the same year by the computations of M. Nicolai of Manheim, 



JUPITER. 143 

relative to another of tlie minor planets — Juno. The most ac- 
curate method of determining the mass of Jupiter, viz., by ob- 
serving the elongations of his fourth or most distant satellite, 
had not been put into practice at this time with such improved 
means as modern observatories afford; but, about the year 
1834, Mr. Airy undertook a series of observations at Cambridge, 
which established the ratio of the masses, as 1048 to 1. The 
latest researches on this subject are those of Professor Bessel of 
Konigsberg, who, in an extensive course of observations on the 
satelhtes, concluded that the Sun's mas| exceeds that of Jupiter 
1 047*87 times, and this result is now very generally adopted. 

The most ancient observation of Jupiter which we are ac- 
quainted with is that reported by Ptolemy in Book x. chap. 3, 
of the Almagest, and considered by him free from all doubt. 
It is dated in the eighty-third year after the death of Alexander 
the Great, on the 18th of the Egyptian month Epiphi^ in the 
morning, when the planet eclipsed the star now known as d 
Cancri. This observation w^as made on the 3d of September, 
B.C. 240, about 18h. on the meridian of Alexandria. 

A similar occultation of a star by Jupiter was witnessed by 
Pound on the morning of the 22d of IS'ovember, 1716, when a 
Geminorum, or, as it is more usually termed, Castor, was 
eclipsed by the planet. 

The tables in use at present for predicting the places of this 
planet were calculated by M. Bouvard of Paris, on the theory of 
Laplace. They are quite exact enough for all practical purposes ; 
but we have the means of improving them when necessary. 

Very elaborate tables for predicting the phenomena of Ju- 
piter's satellites have been calculated by several astronomers. 
Those now employed in the computation of the nautical ephem- 
erides of Great Britain, France, and Prussia, w^ere constructed 
by the late Baron Damoiseau, and published by the French 
Board of Lonsfitude in 1836. 



CHAPTER X. 

SATUEN". ^ 

npiIE next planet in order of distance from the Sun is Saturn, 
-*- one of the most interesting of the heavenly bodies, as pre- 
senting an extraordinary manifestation of creative power and 
wisdom. He is attended by no fewer than eight satellites, and 
is moreover surrounded by several luminous rings^ which must 
reflect considerable light on some parts of the planet's surface. 

The sidereal revolution of Saturn occupies about 10,759 
mean solar days, or 29^- of our years. His mean distance is 
909,028,000, but in consequence of the eccentricity of his orbit 
the distance from the Sun varies between 857,986,000 miles 
and 960,070,000 miles, or the planet is nearer the Sun by more 
than 102,000,000 miles, when it is situate in 89° longitude, 
where the perihelion point falls, than when it is located in the 
opposite part of the ecliptic. 

The figure of Saturn appears to be a perfect ellipse, though 
it has long been supposed to resemble a parallelogram, " with 
the four corners rounded off, so as to leave both the equatorial 
and polar regions flatter than they w^ouldbe in a regular spheri- 
cal figure." This motion was first advanced by Sir W. Herschel 
after examining the planet in reflecting telescopes of ten, tw^enty, 
and forty feet focal length. Actual micrometric measures by 
Professor Bessel in 1833 gave results unfavorable to this de- 
viation from an elliptical outline, and a recent series of observa- 
tions at our Royal Observatory, by the Rev. R. Main, has fully 



SATURN, 145 

confirmed the measures of the German astronomer. It may be 
stated, therefore, that the form of Saturn does not deviate visi- 
bly from an ellipse, the major axis of which, in the direction of 
the planet's equator, subtends an angle of 17-053^' at Saturn's 
mean distance, while the minor axis is seen under an angle of 
15'394'^ The compression is thus found to be 1-1 0*2 8th or 
the equatorial diameter is to the polar as 1000 to 903. These 
numbers depend on Professor Bessel's observations with the 
grand heliometer at Konigsberg. The Greenwich measures 
make the ellipticity 1-9* 2 3d, or one tenth greater than Profes- 
sor Bessel's. Many years ago Mr. Airy showed that the Her- 
schehan form of Saturn could not be accounted for by theory ; 
and it is undei'stood that Sir John Herschel, after a considera- 
tion of the results obtained at Greenwich, has fully admitted 
the necessity of giving up the idea which he had held in com- 
mon with Sir W. Herschel, respecting the unusual figure of the 
planet. The distortion is evidently owang to some optical cause, 
probably connected in some way with the interference of the 
ring ; yet Professor Struve thought the planet Jupiter exhibited 
a similar deviation from an elliptic outline, until he had con- 
vinced himself, by careful measurement, that such was not 
really the case. The true diameter of Saturn at his equator, 
by a mean of the most accurate and recent observations, is about 
77,230 miles ; and if we adopt a mean value for the polar com- 
pression from the measures at Konigsberg and Greenwich, we 
shall find the diameter in the direction of the poles of Saturn to 
be about 69,300 miles. The planet is consequently nearly ten 
times the diameter of the Earth, and exceeds it in bulk nearly 
one thousand times. 

Though belts are frequently observed with good telescopes 
upon the surface of Saturn, they are far more indistinct than 
those of Jupiter. Spots are of rare occurrence. One was seen 
by Sir W. Herschel in June, 1780, for several days, and M. 

7 



146 THE SOLAR SYSTEM. 

Schwabe of Dessau saw one on the 8th of November, 1847, on 
the southern edge of a dark belt, somewhat to the north of the 
equator, projecting a httle into the bright central zone, which 
was sufficiently distinct to allow of micrometrical measures of 
distance from the limbs of the planet. In 1793, Sir W. Her- 
schel saw a quintuple belt, and availed himself of its visibility 
to determine the time of axial rotation of Saturn, which had 
not been previously ascertained. This he accomplished by very 
frequent and careful examination of the appearance of the belts, 
remarking when it was best seen, how far the belts were sepa- 
rated, and making other observations on their figure, &c. ; from 
a combination of which he concluded that the same configura- 
tion recurred after lOh. 16m. 0*4s., which he inferred to be the 
time occupied by Saturn in one rotation upon his axis, or the 
length of a Saturnian day. This great astronomer satisfied 
himself that the belts had undergone no relative change of any 
consequence during the hundred revolutions through which he 
watched them, and farther concluded that the error of the pe- 
riod he had deduced must be veiy much less than two minutes. 
The axis of Saturn is inclined to his orbit 63^ 10', or 61° 50' 
to the plane of the ecliptic. His seasons, therefore, are proba- 
bly more divei^ified than those of Jupiter. Sir William Her- 
schel considered he had sufficient evidence to shbw that the 
planet is surrounded by a very dense atmosphere, an inference 
drawn not only from the changes in the number and appearance 
of the belts from year to year, but depending also on observa- 
tions of the closer satellites, when about to be occulted by the 
planet ; the nearest satelHte was observed to hang upon the 
disc about twenty minutes, and the next satellite about fifteen 
minutes longer than they should have done were there no re- 
fraction. Periodical changes of color in the polar regions of 
Saturn, and the appearance of large dusky spaces of a cloudy 
character in these parts, which Sir W. Herschel repeatedly no- 



SATURN. 14Y 

ticed, were likewise thought to strengthen the supposition of the 
existence of an atmosphere of considerable density. The gen- 
eral color of the planet's surface is a very pale yellow or yel- 
lowish white. 

When Galileo turned his newly constructed telescope upon 
this planet, he saw that the figure was not round as in the 
case of Jupiter, but at first conceived it to be oblong, though 
on further examination he thought the planet consisted of a 
large globe, with a smaller one on each side. Continuing his 
observations, he remarked that this appearance was not con- 
stantly the same, the appendages on each side of the central 
globe gradually diminishing until they vanished entirely, and 
left the planet nearly round, without anything extraordinary 
about it. He informed Kepler of these circuinstan(?^s in No- 
vember, 1610. Hevelius observed Saturn very attentively 
about the year 1655, and described the various phenomena 
which had been noticed by Galileo. Huyghens, who possessed 
telescopes of gi'eater power than those of the Italian astrono- 
mer, was the first who gave a correct explanation of these 
varied appearances, attributing them to a luminous ring sur- 
rounding the globe of Saturn, the greater apparent diameter 
of which he considered to be to that of the o-lobe as nine to 
four. The discovery of the Ring of Saturn was announced by 
Huyghens in his Sy sterna Saturnium^ a small work published 
at the Hague in 1656 ; and many drawings of the supposed 
figure of the planet, taken before the improvements in optical 
micans revealed the true nature of the phenomena which had 
attracted the attention of Galileo, are appended to this account. 
Huyghen's telescopes, though much, better than those of his 
predecessors, were yet of insufficient power to exhibit all the 
phenomena of the ring. The first mention of a division sepa- 
rating it into two concentric rings is usually supposed to be due 
to the celebrated Dominic Cassini, astronomer at the Observa- 



148 THE SOLAR SYSTEM. 

toiy of Paris, soon after the foundation of that establishment 
by Louis XIV., or in the year 1675. But it is on record that 
two English amateurs, Dr. Ball and Mr. W. Ball, of Minehead, 
North Devonshire, had noticed the duphcity of the ring in 
October, 1665, giving them a priority of ten years in this in- 
teresting discovery. 

The ring of Saturn may be described as broad and flat, 
and is situated precisely in the plane of the planet's equator ; 
it is therefore inclined to the ecliptic at an angle of 28^ 10' 27'', 
and intersects it in longitude, 167° 31' 52" and 347^ 31' 52", 
which points are called the ascending and descending nodes of 
the ring respectively. It is owing to this inclination of the 
plane of the ring to the ecliptic and to the orbit of the primary, 
that tbis appendage is sometimes observed as a broad elhpse, 
and at others as a straight line, but just discernible in the most 
powerful telescopes hitherto constructed. For when the helio- 
centric longitude of Saturn is either 167^ 32', or 347° 32', the 
plane of the ring passes through the Sun, which consequently 
can only illuminate the thin edge ; and it is for this reason in- 
visible to ordinary telescopes. It will disappear also when the 
plane passes through the Earth, or when the Sun shines on 
that part of the surface which is turned from us. Generally 
the ring disapj^ears twice, for the motion of Saturn is so slow 
in comparison with that of the Earth, that our globe passes 
twice through the plane of the ring before it is carried past 
the plane of the ecliptic. Thus, in 1848, after the north sur- 
face had been visible nearly fifteen years, the ring became in- 
visible on April 2 2d, when the Earth was in its plane, and the 
Sun above it : it reappeared on the 3d of September, when the 
Sun was in the same plane, passing south to the same side as 
the Earth ; and consequently, when the southern illuminated 
surface w^as turned towards us. On the 12th of the same 
month, the Earth passed to the northern side, while the Sun 



SATURN. 149 

still shone on the southern surface, and the ring therefore dis- 
appeared a second time. It continued to present to us its un- 
illuminated surface until the 18th of January, 1849, when the 
Earth passed to the southern side of the plane of the ring, 
which had been turned towards the Sun since the 3d of Sep- 
tember. We shall continue to see the southern surface until a 
few months before Saturn arrives in the opposite part of his 
orbit, or at the ascending node of the ring, which will take 
place tow^ards the close of the year 1861, and after the occur- 
rence of similar phenomena to those we have just described, 
the northern surface of the ring will become visible, and con- 
tinue so until the year 1877, w^hich may witness a series of 
disappearances and re-appearances similar to those of 1848 and 
1849. After the plane of the ring has passed through the 
Sun, the surface illuminated by him becomes broader and 
broader until Saturn is distant 90^ from the nodes on the 
echptic, or is situate in longitude 257'5^, or 77'5^, or about 
the middle of Sagittarius and Gemini, in which positions we 
see the rings most open, the minor axis being almost precisely 
half the greater one. These times are the most favorable for 
the examination of the division of the ring, which may then 
be traced nearly all round, the only interruption b-'ing caused 
by the globe of Saturn. The belts also appear at their greatest 
curvature, and the shadow of the ring upon the surface of the 
pknet, and of the planet itself upon the ring are distinctly 
visible in good telescopes. The apparent major axis of the 
ring at Saturn's mean distance is about 39^', and the diameter 
of the globe about 17'5'^ consequently the surface of the ring 
covers either the north or south pole of the planet for some 
little time, about the periods just referred to, since, as we have 
observed, the minor axis is then very nearly equal to half the 
major axis, or is nearly 19'5'\ The opposite pole is, then, for 
the same reason, projected upon the surface of the ring. 



150 THE SOLAR SYSTEM. 

We have already noticed the discovery in England and 
France of the double ring of Saturn during the latter half of 
the seventeenth century, and we shall now describe in some 
detail the observations which have been made by various as- 
tronomers tending to prove that the Ring is at least a triple 
one, and possibly may be more correctly termed multiple. 
M. Lalande mentions, in the last edition of his " Astronomic," 
that the well known English optician, Mr. Short, had informed 
him orally of his having noticed the oute?' ring of Saturn 
divided by several black lines : this was with a telescope of 
twelve feet focal length. M. Laplace, in his " Theory of Sat- 
urn's Ring," published in the memoirs of the Paris Academy 
for 1Y87, mentions the same circumstance, and infers that the 
outer Ring must be formed of several smaller ones nearly in 
the same plane, — that of the planet's equator; in which situa- 
tion he concluded they are retained by the attraction of the 
equatorial parts, which, as we have already seen, are more 
prominent than the polar regions. M. Lalande gives a figure 
representing the appearance noticed by Mr. Short. Both M. 
Delambre and M. Biot, in their treatises on Astronomy, refer 
to these divisions ; but it does not appear that they had any 
further evidence than is furnished by M. Lalande. The ob- 
servations of Mr. Short were probably made about the year 
1760. In a paper read before the Royal Society of London 
in December, 1791, Sir W. Herschel mentions having seen, be- 
tween the 19th and 26th of June, 1780, "a second black list" 
upon the ring, close to the inner side, and on the preceding 
arm only. This appearance was visible with three reflecting 
telescopes, and could hardly arise, therefore, from optical illu- 
sion. On the 29th of the same month, no such division of the 
inner ring was perceptible. The visibility of a dark line on one 
side only, Sir W. Herschel thought might be accounted for by 
supposing the opening very narrow and the rings eccentric-. 



SATURN, 151 

In December, 1823, Professor Quetelet, at Paris, noticed 
the outer Ring of Saturn divided into two, with an achromatic 
telescope of ten inches aperture. This circumstance is related 
by Captain Kater in Vol. iv. of the '' Memoirs of the Royal 
Astronomical Society," where he has also given the results of 
some observations of his own, which tend to prove the exist- 
ence of such divisions. On the l7th of December, 1825, with 
a Newtonian reflector by Watson of six inches aperture and 
forty inches focus, the outer ring " appeared separated by 
numerous dark divisions, extremely close, one stronger than 
the rest dividing the ring about equally. The inner ring de- 
cidedly -had no such appearance." It is added that this phe- 
nomenon was noticed only with Watson's Newtonian. On the 
16th of January of the following year, Saturn was examined 
by Captain Kater, w^ith a telescope of the same construction 
by DoUond ; the outer ring was thought to be made up of 
several, as before mentioned, but it was not so distinct as with 
the smaller telescope, on December l7th. On the following 
night, "the outer ring appeared to be made up of several 
rings," but not very distinctly marked, so that some doubt at- 
tached to the observation. On the 22cl January, 1828, Cap- 
tain Kater could see no trace of divisions in the exterior ring, 
and thence concludes that they are not permanent. 

On the 28th of May, 1837, Professor Encke, observing 
with the great telescope of Fraunhofer at Berlin, not only saw 
the outer ring of Saturn divided by a black line, but obtained 
micrometrical measures of the diameter of the division. It 
was found that the .outer diameter of the outer ring was 
40*445'', reduced to Saturn's mean distance ; the diameter of 
the new division 37*471'', and the inner diameter of the ex- 
terior ring 36*038". Hence it would appear that the exterior 
ring is not equally divided. On April 25, Professor Encke 
had seen the extra division, but could not measure it. 



152 THE SOLAR SYSTEM. 

In 1838, M. Dumouchel, Director of the Observatory of 
the Collegio Romano at Rome, published an account of some 
new divisions in the ring of Saturn which had been noticed by 
M. de Vico with the large achromatic telescope of that estab- 
lishment. Having heard of Professor Eocke's observations, 
M. de Vico took advantage of some very fine nights in the 
summer of 1838 to scrutinize the appearance of the planet. 
On the evening of May 29, he very distinctly saw, and showed 
to several pupils and friends, besides the two principal rings, 
three other divisions or black lines, the one nearly in the mid- 
dle of the exterior ring, and two upon the interior one. It is 
stated that the observations of following days indicated some 
variation in the number of zones, according as the sky was 
more or less favorable. About the time of meridian passage, 
sometimes as many as six rings were noticed, the distinction 
being such that it was difficult to admit any optical illusion as 
the cause of the appearance. 

M. Schwabe, of Dessau, paid particular attention to the 
phenomena of Saturn's ring in the summer of 1841, employ- 
ing in his examinations a six-feet achromatic telescope by 
Fraunhofer. On July 26th, soon after 9h., Encke's division 
was seen by glimpses, with powers of about 290 and 360. On 
August 10th, it was just perceptible on the eastern ansa, and 
again on the I7th. On September 10th, this division was 
noticed at the western side. M. Schwabe observed on thirty 
days ; but the extra division was noticed on four occasions only, 
as above. 

In the month of September, 1843, a very satisfactory ob- 
servation of the division in the exterior ring was made by Mr. 
Lassell and the Rev. W. R. Dawes, with a nine-feet Newtonian 
reflector constructed by the former gentleman. On the Yth of 
that month, the sky at 9 p.m. hazy and the stars dull, the tel- 
escope was turned upon Saturn, and under a power of 450 the 



SATURN. 153 

outer ring was distinctly perceived to be divided into two. The 
outline of the planet was very sharply defined with this power, 
and the primary division of the ring was very black, and stead- 
ily seen all round the southern side. When this was most sat- 
isfactorily observed, a dark line was pretty obvious on the outer 
ring. Mr. Dawes was not only perfectly satisfied of its existence, 
but, during the best views, obtained some estimations of its 
breadth in comparison with that of the ordinary division. The 
proportion appeared to him to be as 1 to 3 ; but Mr. Lassell 
considered it scarcely one third. Both observers, however, 
agreed in placing it outside the middle of the exterior ring. It 
was equally visible at both ends, Mr. Dawes adds, that neither 
he nor Mr. Lassell had any glimpses of further subdivisions. 
" The shading of the inner ring was very obvious, but no dark 
line was even suspected in that situation." Such evidence of 
the reality of the divisions in the exterior ring as is afforded by 
the observations of Messrs. Dawes and Lassell is conclusive 
enough, and it is fortunate that we have the testimony of these 
able observers in a question of such delicacy. 

Professor Challis obtained some glimpses of this extra di\'i- 
sion in 1842 and 1845, with the great Xorthumberland teles- 
cope at Cambridge, and about the middle of September, in the 
latter year, the author saw what he considered to be Encke's 
division on the ea.stern arm of the ring. More recently, Mr. 
Lassell and the Rev. W. R. Dawes have had most satisfactory 
views of a dark line on the outer ring, but not exactly in the 
position indicated by Encke's measures. 

The most recent, and, at the same time, one of the most 
remarkable discoveries in reference to the rings of Saturn, is 
that of a dusky or obscure ring nearer to the planet than the 
interior bright one. It appears that Dr. Galle of Berlin had 
noticed a gradual shading off" of the inner ring towards the globe 
of Saturn, and had published measures of the extent of the 



154 THE SOLAR SYSTEM. 

darker part in the Transactions of the Berlin Academy m 1838. 
The memoir, however, was but little known, for in 1850, after 
the reappearance of the ring in a position favorable to the ob- 
servation of the divisions, the dusky zone was remarked as new 
by Mr. Bond of Cambridge, United States, and by the Rev. W. 
R. Dawes at Wateringbury, near Maidstone, about the same 
time (in the month of November). Mr. Bond, we beheve, has 
seen only that portion of the obscure rino- which had been pre- 
viously noticed by Dr. Galle at Berlin, but the English astron- 
omer, ivith his excellent eye and instrument, has succeeded in 
making out some additional facts respecting this wonderful ap- 
pendage. He sees not only that there is a dusky ring near the 
interior edge of the inner bright one, but that it is certainly a 
double one, being divided during the most favorable views by 
an extremely fine line. Supposing with Professor Struve that 
the interval between the globe of Saturn and the interior bright 
ring is 4'34^', Mr. Dawes estimates the breadth of the different 
portions of the obscure ring as follows : — 

Interval between the bright ring and exterior obscure ring . 0'3'' 

Breadth of the exterior obscure ring 1*1" 

Breadth of interior obscure ring, and of the dark boundary 

separating the two 0*6^' 

By numerous measures of the breadth of the dark ring, includ- 
ing the whole space between the inner edge of the bright ring 
and the inner edge of the interior obscure one, the angle sub- 
tended was found to be 1*94''. The more distant portion of 
the new ring is seen much more distinctly than the correspond- 
ing portion nearest to the Earth. The projection of this ring 
upon the ball was noticed by the Rev. W. R. Dawes in No- 
vember, 1850, and as might be expected from that eminent 
observer, the connection of the dark line crossing the disc, and 
broader at the edges than towards the centre, with the obscure 



SATURN. 



155 



portion of the ring seen on each side of the planet, was imme- 
diately discovered. 

The follo\Ying table exhibits the dimensions of Saturn's nngs 
in equatorial semi-diameters of the primary, and also in Eng- 
lish miles. They are calculated from the most accurate micro- 
metrical measures hitherto published, and must be pretty near 
approximations to the true values : — 



Outer diameter of outer ring 
Inner diameter of outer ring 
Outer diameter of inner ring 
Inner diameter of inner ring 
Breadth of outer ring 
Breadth of inner ring . 
Breadth of the principal division 
Distance of the inner ring (inte 
rior edge) from Saturn's limb 
The same from Saturn's centre , 



orial semi-diameter 


3. Englisli miles. 


4-4575 


172,130 


3-9232 


151500 


3-8326 


148 000 


2-9648 


114,480 


. 0-26715 


10,.316 


. 0-43391 


16 755 


n 0-04536 


1;752 


. 0-48238 


18,628 


. 1-48238 


57;243 



Sir John Herschel estimates that the thickness of the rings 
does not exceed a hundred miles. Sir W. Hei'schel was con- 
vinced it must be very much less than the diameter of the 
smallest satellite, which he judged might be about one thousand 
miles. 

The diameter of Encke's division, according to the obseiwa- 
tions of that astronomer, is 4'277 8 equatorial radii of Saturn, 
or 165,100 miles ; it is therefore situated at a distance of about 
3500 miles from the exterior edge of the outer ring. 

The planet Saturn is attended by eight satelhtes, seven of 
■which revolve in orbits lying nearly in the plane of the ring, 
and consequently of the planet's equator. 

A good deal of confusion having arisen in the nomenclature 
of these satellites. Sir John Herschel proposed in 1847 a seiies 
of mythological names to distinguish the seven satelhtes then 



156 'J^HE SOLAR SYSTEM. 

known, and the faint one more recently disco\ ered having also 
received a classical name, we shall adhere strictly to the plan 
adopted by this eminent astronomer, as there can be no doubt 
these moons will be known hereafter by the names Sir John 
has assigned them. Taking the satellites in order of distance 
from Saturn, their names will be — Mimas, Enceladus, Tethys, 
Dione, Rhea, Titan, Hyperion, and Japetus ; but in our descrip- 
tion of them we shall follow the order of discovery. 

Titan, the great satellite of Saturn, was discovered by Huy- 
ghens on the 25th of March, 1655, with telescopes of twelve 
and twenty-three feet focal length. He observed it attentively, 
and published tables of its movements in 1659, in his Systema 
Saturnium. These tables were afterwards improved by Hal- 
ley, Cassini, and Lalande. But the most exact determination 
of its orbit is that recently published by the late Professor Bes- 
sel of Konigsberg, which depends on his own observations 
with the fine heliometer at the observatory at that place. The 
period of one sidereal revolution is 15d. 22h. 41m. 24*86s., and 
the mean distance from the centre of the planet 176*55", or 
7 78,000 miles. This satellite shines with the brilliancy of stars 
of the eighth magnitude, and in powerful telescopes exhibits a 
very decided disc. 

The next satelhte in order of discovery is Japetus, the most 
distant of all, detected by the elder Cassini at Paris at the end 
of October, 1671, with the aid of a telescope of seventeen feet 
focal length. The period of a sidereal revolution, according to 
the latest investigations, is 79d. 7h. 54m. 40*8s., and the semi- 
axis of the orbit subtends an angle of 514*52'' at Saturn's mean 
<listance from the Earth ; whence we find the distance of the 
wittellite from the centre of the planet to be 2,268,000 miles, 
which is nearly twice that of the furthest satellite of Jupiter. 
The plane of the orbit of this satellite is not nearly coincident 
with the plane of the ring, and consequently of Saturn's equa- 



SATURN. 157 

tor, as in the case of the other satellites, but is inclined thereto 
at an angle of 10^, the node being placed in 150° 27' longi- 
tude upon the orbit of the planet according to the calculations 
of Lalande. Cassini found the longitude of this point 155° ; 
but it appears certain that there is a retrograde motion of the 
line of nodes, though we are as yet without any precise deter- 
mination of its amount. 

Cassini noticed that this exterior satellite regularly disap- 
peared during half its revolution, when to the east of Saturn, 
or following the planet in right ascension. Hence he concluded 
that it revolved upon its axis in the time of revolution round 
the primary, as in the case of our Moon. Sir J. Newton in his 
Principia also expressed the same opinion, and thought the 
variations in the degree of brightness of the satellite was owing 
to some parts of its surface being less capable of reflecting the 
Sun's light than others. Sir WiUiam Herschel traced the fluc- 
tuations of light daring more than ten revolutions of Japetus, 
and found that the same phenomenon always recurred when 
the satellite returned to the same position in its orbit. At max- 
imum brightness Japetus appears like a star of the ninth mag- 
nitude, but more usually the light is not greater than that of a 
star of the tenth or eleventh class. 

The satellite which Sir John Herschel has called Rhea was 
detected by Cassini at Paris, on the 23d of December, 1672, 
with the help of telescopes of 35 and 70 feet focal length. Its 
period of revolution is found to be 4d. 12h. 25m. ll'ls., and its 
distance from the primary 336,000 miles, which subtends an 
angle of 76*16'' when Saturn is placed at his mean distance 
from the Earth. The plane of the orbit of this satellite is very 
nearly coincident with that of the plane of the ring, so that 
about those times when the primary is near the nodes of the 
ring, the orbit will appear to be a straight line ; but at all other 
periods the apparent form of the orbit is an ellipse, which is 



158 THE SOLAR SYSTEM. 

least eccentrical at the time that the ring is most open, or when 
Saturn is 90^ distant from its line of nodes. Sir John Her- 
schel has investigated the elements of Rhea from his own ob- 
servations, taken with a twenty feet reflector, during his resi- 
dence at the Cape of Good Hope, in the years 1835-7. The 
greatest equation of the centre appears to be 2° 36', correspond- 
ing to an eccentricity of 0*02269, the perisaturnium being 
placed in 95° longitude. These numbers, however, will be only 
approximate. Generally speaking, this satellite shines like a 
star of the tenth or eleventh magnitudes, but at times it will 
more nearly resemble one of the ninth, and at others one of the 
twelfth magnitudes. Much depends on the position in respect 
to the primary, and on atmospheric conditions. To see it stead- 
ily at any time, an instrument of not less power than the ordi- 
nary five feet achromatic should be employed. 

Dione was discovered by Cassini in March, 1684, with 
lenses varying from 34 to 220 Parisian feet in focal length. 
The period is 2d. I7h. 44m. 51'2s., and the distance from the 
centre of the primary 240,000 miles, subtending an angle of 
54*54''. It is not so easily seen as Rhea^ but will occasionally 
equal in brightness a star of the eleventh magnitude, though far 
more commonly it resembles one of the twelfth, or is even, 
fainter still. A powerful instrument is therefore required to 
observe it satisfactorily. The orbit of Dione is stated by Su* 
John Hei-schel to be elliptical, the greatest equation of the 
centre being 2^ 22', and the position of the perisaturnium in 
longitude 42° 30' for the year 1836. The satelHte is supposed 
to revolve in the plane of the ring ; the same remarks in regard 
to the apparent form of the orbit that w^ere made in the last 
article will therefore apply here, and with respect to all the 
other satelUtes except Japetus. 

Tethys was likewise detected by Cassini about the same 
(ime as Dione, or in March, 1684, with the same telescopes, or 



SATURN. 159 

with lenses mounted without tubes in consequence of their great 
focal length. Her period of revolution round the primary is 
Id. 21h. 18m. 25-9s., at a mean distance of 188,000 miles, 
which subtends an angle of 42*57'^ at the average distance of 
Saturn from the Earth. Generally this satellite resembles a 
star of the thirteenth mao-nitude, and can therefore be well ob- 
served only in powerful telescopes. Dr. Lamont, of Munich, in- 
vestigated the elements of the orbit from his own observations 
in 1836 ; he found that the eccentricity was 0*0051, giving the 
greatest equation of the centre 0^ 35^ ; the place of the perisa- 
turnium in 1836 was 35 7^ 39^, the longitude of the ascending 
node 184^ 36\ and the inclination of the orbit to the plane of 
the ring 1° 33', the longitude of the satellite in this orbit (re- 
ferred to the same plane) on the 23d of April, 1836, at 8h. 
27m., Greenwich mean time, being 158^ 31'. Sir John Her- 
schel made a series of observations at the Cape of Good Hope 
about the same year, from which he inferred that the eccen- 
tricity of the orbit of Tethys amounted to 0*04217, giving the 
greatest equation 4^ 50' ; the perisaturnium was fixed in longi- 
tude 53^ 40', and the satellite was assumed to revolve exactly 
in the plane of the ring. The differences between the calcula- 
tions of these astronomers are to be attributed to the difficulty 
attending the observations, and to the circumstance of small 
errors of measurement in the apparent position of the satellite 
with respect to the planet greatly affecting the deduced ele- 
ments. It will be remarked that the three female names (Rhea, 
Dione, and Tethys) selected by Sir John Herschel serve to dis- 
tinguish the three sateUites discovered by the elder Cassini. 

Unceladus^ one of the closer satellites of Saturn, was first 
perceived by Sir William Herschel, on the evening of August 
19, 1787, about its greatest western elongation; but the dis- 
covery was not confirmed till the completion of the forty-feet re- 
flector in August, 1789. The very moment this instrument was 



160 ^^-^ SOLAR SYSTEM. 

first directed to the planet, on the 28th of that month, six sat- 
elUtes Tvere seen, " in such situations and so bright, as rendered 
it impossible to mistake them." Sir W. Herschel says, this 
new satellite is not so conspicuous as the interior of Cassini's 
(Tethys). It is visible only in the most powerful telescopes to 
be found in observatories. Professor Struve states that with 
the Dorpat Refractor of nine inches aperture, the five old satel- 
lites were readily distinguished in an illuminated field, but En- 
celadus had only been caught occasionally in a dark field. Mr. 
Lassell says it is instantly seen in his twenty-foot reflector, un- 
der all tolerable circumstances, when within 40^ or 50^ of his 
gi'eatest elongation. Sir John Herschel, on several occasions, 
has estimated it of the fifteenth magnitude. The author be- 
lieves he has seen it more than once with a telescope ha^-ing an 
object glass of seven inches aperture, but with this instrument 
it was only caught by glimpses, and could not have been prop- 
erly observed. Hence we may conclude, that a telescope of less 
than five inches aperture will stand but little chance of showing 
this satellite at all. 

By the observations of Sir John Herschel at the Cape of 
Good Hope, and those at Slough in 1789, the period of a side- 
real revolution appears to be Id. 8h. 53m. 6*8s. From the ob- 
servations of Sir W. Herschel alone, M.M. Beer and Madler find 
a period of Id. 8h. 53m. 2*7s., and consider the orbit as circu- 
lar in the plane of the ring, the saturnicentric longitude of the 
satellite on the 14th of September, 1789, at lib. 53m. mean 
time at Slough, being 67^ bQ' 26^'. The distance of Encela- 
dus from the centre of the primary is 152,000 miles, subtending 
an angle of 34*38^^, when Saturn is at his mean distance from 
the Earth. 

Mimas, the closest satellite, was discovered by Sir W. Her- 
schel, with his forty -feet reflector, on the l7th of September, 
1789. Even with this grand instrument it is described as a 



SATURN. 161 

" very small lucid point." It is the faintest object imaginable, 
as may be judged from the fact, that Sir John Herschel never 
saw it more than once with his reflecting telescope of twenty feet 
focal length, employed in his great survey of the heavens re- 
cently completed. Mr. Lassell has been more fortunate, but 
states that the difference in the degree of visibility of Enceladus 
and Mimas is almost incomparable, and under all but the most 
favorable conditions, the latter is an object of extreme difficulty. 
It is only the giant telescopes that are powerful enough for ob- 
servations of the satellites, and there are probably very few in- 
struments of less than seven inches aperture, which could give 
the least indication of it.^ 

M.M. Beer and Madler have inferred from Sir W. Herschel's 
observations in 1789, that the orbit is eUiptical, the greatest 
equation of the centre being 7° 54', and the position of the 
perisaturnium 104*42°. The period they assign is 22h. 36m. 
lY'705s., and the longitude of the satellite, as seen from Saturn 
on the 14th of September, 1789, at 13h. 26m. mean time at 
Slough, appears to have been 264° 16' 36". Hence we find 
the real mean distance of the satellite from the centre of the 
primary to be 118,000 miles, subtending an angle of 26*78". 
When Mimas is at his greatest elongations, he appears about 
half the length of one of the ansae of the ring from its ex- 
tremity. 

The eighth sateUite of Saturn, which has received the name 
Hyperion^ was discovered nearly simultaneously, by Mr. Lassell, 
at Liverpool, and by Professor Bond, of Cambridge Observa- 
tory, United States. Mr. Lassel caught his first glimpse of the 
satellite on the 18th of September, 1848, not far from the po- 

* The Eev. W. R. Dawes, with his excellent Munich refractor of 
6^ inches aperture, has more than once obtained a favorable view of 
Mimas ; but then it must be remembered that this gentleman is one of 
the most practised observers of delicate objects that Europe affords. 



162 THE SOLAR SYSTEM. 

sition of Japetus, and being uncertain whether it was that satel- 
lite or not, he made a drawing of the small stars surrounding 
Saturn for re-examination on the next clear evening. On the 
19th he was astonished to find that both stars had moved 
away from the positions they had occupied on the previous 
night, one of them having moved northward in conformity with 
the orbital movement of Japetus, while the fainter of the two 
remained in the line of the interior satelHtes, and seemed to 
have drawn rather closer to the planet. Mr. Lassell was able 
to establish the fact of his having discovered a new satellite on 
the same evening, for he perceived a very sensible change in its 
position relative to the stars, which was decisive as to the nature 
of the object. Professor Bond saw this satellite for the first 
time on the 16th of September, and describes it as a point of 
light, resembling a star of the seventeenth magnitude, in the 
plane of Saturn's ring. A diagram was made for comparison 
on another evening. On the 18th this object was seen and re- 
corded again "with a doubt expressed as to its character." On 
the 19th (the same night that Mr. Lassell verified his discovery) 
Professor Bond ascertained that the small star partook of the 
retrograde motion of Saturn, and must consequently be a new 
satellite. Thus it appears that the discovery of this body should 
date from the evening of September 19, 1848, when its true 
nature was first determined by the English and American 
astronomers. We have but few instances of so close and re- 
markable a coincidence in the first detection of a heavenly body. 
The elements of Hyperion have not yet been accurately as- 
certained. It appears probable, however, that the period of 
revolution round the primary does not differ much from 2 Id. 
4h. 20m., the apparent mean distance from the centre of Saturn 
being 213*3'', indicating a real distance of about 940,000 miles. 
The orbit seems to be more eccentrical than in the case of any 
other satelHte, the perihelion point falling in about 2 95 "^ longitude. 



SATURN. 163 

With regard to the diameters of the satellites of Saturn we 
know but little. Titan^ the most distant but one of the eight, 
is however by far the largest. Sir W. Herschel saw a pretty 
considerable disc with a power of 500, and remarked that it 
appeared reddish. Professor Struve has also seen it as a small 
round disc about 0*7o'' in diameter, with the great telescope 
at the observatory of Dorpat in Russia. This estimation would 
give us 3300 miles for the real diameter of Titan, which is, 
perhaps, not far from the truth. The ruddy tinge may be in- 
dicative of an extensive atmosphere. The diameter of the 
closest satellite, Mimas, was judged by its discoverer to be about 
1000 EnHish miles, and he remarked that Enceladus was larg-er 
than Mimas, but not quite equal to Tethys. The exterior satel- 
lite, Japetus, is next in size to Titan, and much larger than the 
interior ones. Schrotef gave measures or estimations of the 
diameters, from which it would appear that Titan is 2850 miles, 
Japetus 1800 miles, Rhea 1200 miles, and Dione and Tethys 
500 miles in diameter ; the numbers for the closer satellites 
must be received with caution. 

Sir John Herschel has pointed out a curious relation between 
the periods of the four interior. sat eUites of Saturn. The time 
of revolution of Mimas is half that of Tethys^ and the period of 
Enceladus half that of Dione. 

The eclipses of the satelhtes are only visible to us at those 
times when the Earth is near the plane of the ring, and even 
then, as will be readily supposed, the most powerful telescopes 
would be required to observe them with any degree of certainty. 
Occultations of the satellites by Saturn were occasionally ob- 
served by Sir William Herschel, and one satellite has been seen 
to echpse another, or aj^proach so close to it that no dark line 
could be seen between them. These conjunctions of the satel- 
lites were frequently witnessed by the eminent astronomer just 
named. The interior satellites were seen projected on the ed:-;e 



164 THE SOLAR SYSTEM. 

of the ring at the time the Earth was in -its plane in the year 
1789, and Sir W. Herschel made use. of them as "microme- 
ters," by which to estimate the thickness of the rings, his ob- 
servations leading him to the conclusion that it was very much 
less than the diameter of either Mimas or Enceladus. 

On the 2d of November, 1789, Sir W. Herschel observed 
a transit of the shadow of Titan over the disc of the planet. 
It appeared as a black spot, darker than the equatorial belt, 
and was traced from the south preceding edge of the disc up to 
Saturn's centre, where it arrived in about 2h. 10m. after it was 
first perceived on the planet's surface. 

It has been notified to the American Academy of Arts and 
Sciences that Professor Peirce was about to enter on an inves- 
tigation of the motions of the satellites, at present a great de- 
sideratum. It would materially assist the practical astronomer 
were he in possession of tables of the satellites, sufficiently ex- 
act to enable him to predict their phenomena within anything 
like reasonable limits of error. 

The most ancient observation of Saturn which has descended 
to us was made by the Chaldeans, probably at Babylon, in the 
year 519 of Nabonassar's period, on the 14th of the month 
J'^/5^, in the evening, when the planet was observed to be two 
digits below the star in the southern wing of Virgo, known to 
us as y Virginis. The date given by Ptolemy, who reports 
this observation in his Almagest, answers to b.c. 228, March 1. 

An occupation of Saturn by the Moon was observed as 
early as the year a.d. 503, at Athens. On the 21st of Feb- 
ruary, at llh. 44m. p.m., the planet was seen emerging from 
the middle of the illuminated hmb of the Moon. The obser- 
vation is mentioned by Ismael Bullialdus in his Astronomia 
Philolaica, and was copied from a Greek manuscript at that 
time, preserved in the Bihliotheque du Eoi at Paris. 



CHAPTEE XL 

URANUS. ^ 

PREVIOUS to the year 1 7 81, the only planets known besides 
the one we inhabit were Mercury, Venus, Mars, Jupiter, 
and Saturn, all which are more or less conspicuous to the naked 
eye, and were recognized as wandering bodies from the earliest 
antiquity. Saturn was supposed to be the most distant mem- 
ber of the solar system, and very little suspicion of the exist- 
ence of any exterior planet was entertained. The close exami- 
nation of the heavens, commenced by Sir W. Herschel with 
his powerful telescopes in 1779, led however to a most remark- 
able discovery, which almost doubled the extent of our system. 
On the 13th of March, 1781, between ten and eleven 
o'clock in the evening, while engaged in examining with 
his seven-feet reflector the small telescopic stars, near a 
brighter one called H in Gemini, Sir William noticed one 
which appeared visibly larger than the rest, and being struck 
with this circumstance, he apphed high magnifying powers, 
with a view of ascertaining the true nature of the object, for 
the fixed stars are not proportionally magnified with high pow- 
ers, as are the planets and comets. The diameter w^as so much 
increased beyond what that of a fixed' star should have been, 
that Sir W. Herschel immediately conjectured it to be a comet, 
and it was actually announced as such to the Royal Society on 
the 26th of April. Perhaps one reason which induced this 
great astronomer to adopt this conclusion was, the apparent 



166 THE SOLAR SYSTEM. 

regular increase in the measured diameter of the object between 
March 13 and April 18 ; but we are now certain that this grad- 
ual increase must have been owing to some optical deception. 
At any rate, Sir William seems to have had no idea at this time 
of the real nature of his discovery. 

On March lY the new star was observed by Dr. Maskelyne, 
then Astronomer Royal, who is understood to have immediately 
expressed his suspicion of its planetary nature. It was seen by 
M. Messier, at the Paris Observatory, a month later, or on April 
17, and at Berlin by Professor Bode, on July 18, after which it 
^^ecarae an object of engrossing attention at all the observato- 
ries of Europe. 

As soon as a sufficient number of positions had been ob- 
tained, astronomers attempted to represent them by a parabolic 
orbit, under the idea that they belonged to a comet situated at 
a great distance from the Sun. The assumption being found 
irreconcilable with the observations, M. Lexell and others cal- 
culated the orbit without this hypothesis, and speedily came to 
the conclusion that its true form differed but little from a circle, 
the radius of which was about nineteen times the Earth's mean 
distance from the Sun. M.M. Hennert, Mechain, Lalande, and 
the President de Saron arrived at very similar- results. The 
stranger was then recognized as a superior planet, next in order 
of distance to Saturn, and it was inferred that the period occu- 
pied in one revolution round the central luminary must be about 
eighty -two years. The observations of the first period of visi- 
bility, before the conjunction of the planet with the Sun, were 
sufficient to prove that the plane of its orbit deviated but shghtly 
from that of the ecliptic, and that no very great eccentricity 
could exist. 

The reader must not imagine from what has been said of 
the manner in which the planet was discovered, that we are 
indebted to accident for this addition to the members of the 



URANUS. 167 

solar system. It is very likely true that little notion of finding 
an exterior planet was entertained by Sir AY. Hei^schel, when 
he comraenced his survey of the heavens, yet it must be re- 
membered that this great astronomer was at work on a system- 
atic plan^ which was followed up with a zeal and diligence no 
less the admiration of his contemporaries, than an example in 
after times. 

The name for the new primary planet was a subject of some 
contention amongst astronomers. Sir W. Hei-schel, to whom 
belonged the right of selecting a name, termed it the " Georgi- 
um Sidus^'' in gratitude to George III., the munificent patron 
of the science. Continental astronomers were not disposed to 
receive any but a mythological name, and Cybele, Atlas, Xep- 
tune, &c., were suggested. Professor Bode soon afterwards 
proposed Uranus (the father of Saturn), and this name gradu- 
ally gained ground, notwithstanding a good deal of opposition. 
Laplace insisted upon calHng the planet Herschel in compli- 
ment to the eminent observer who had brought it to light, and 
in this he was followed by many cultivators of astronomy in 
this and other countries. The term '' Georgian'''' was substi- 
tuted by some persons in place of the longer name assigned by 
the discoverer ; and the planet has been thus styled in our 
Nautical Almanac^ until the appearance of the volume for 
1851, when it was rejected in favor of Bode's appellation 
" Uranus," which for a long time past has been in universal 
use amongst astronomers. This change we believe to have 
been made with the full consent of Sir John Herschel, the son 
of the great discoverer. » 

The theory of the motions of Uranus occupied the atten- 
tion of the celebrated French geometer Laplace very soon after 
the detection of the planet in 1781. Various inequalities of 
long period were discovered, the amount of ellipticity was de- 
termined, and the position of the lines of nodes and apsides. 



168 THE SOLAR SYSTEM. 

Fixmillner, Delambre, and other eminent calculators, also occu- 
pied themselves with the numerical elements and formation of 
tables, in order to predict the future positions of the planet. 
In these computations they were not confined to observations 
taken since the actual discovery of Uranus by Sir W. Herschel. 
As soon as an approximation to the orbit had been obtained, it 
became a matter of great interest and importance to ascertain 
if Flamsteed or other observers in previous times had cata- 
logued the planet as a fixed star, for if this were the case, the 
date of the observation being recovered, the position would 
prove of the utmost value in assigning the exact elements of 
the planet's orbit. A careful search was made with this object 
in view, and it was found that the planet had been repeatedly 
observed by Flamsteed at our Royal Observatoiy, and by Le- 
monnier at Paris, and was once observed by Tobias Mayer at 
Gottingen. Flamsteed saw the planet first on the 13th of De- 
cember, 1690, and entered it in his catalogue as the 34th star 
in Taurus. He observed it also on March 22, 1*712, and on 
February 21, 22, 27, and April 18, 1Y15. Mayer took its 
transit over the meridian of Gottingen, on September 25, lYoG. 
Lemonnier observed it no less than twelve times, and had he 
reduced his observations as he made them, would in all proba- 
bility have detected the planet. The dates are October 14, and 
December 3, 1750, January 15, 1764; December 27 and 30, 
1768; January 15, 16, 20, 21, 22, and 23, 1769; and De- 
cember 18, 1771. Thus, the planet Uranus had been observed 
as a fixed star at least nineteen times before its real nature was 
detected by Sir W. Herschel. 

The length of a sidereal revolution of Uranus is about 
30686*7 days, or rather more than 84 of our years, according 
to the recent researches of M. Le Yerrier. Its mean distance 
from the Sun is 1,828,071,000 miles, the least distance being 
1,742,738,000, and the greatest 1,913,404,000, so that the 



rRAxrs. 169 

eccentricity causes a variation in length of the radius-vector of 
about 170,666,000 miles. The plane of the orbit is very nearly- 
coincident vrith the ecliptic, the inclination being less than 47'. 

The apparent diameter of the planet is subject to very little 
change during the time it is visible from the earth, and hardly 
exceeds four seconds of space, but at a distance = 1, this diam- 
eter Avould subtend an angle of 78'4^\ The real diameter of 
Uranus is therefore about 35,000 miles. Professor Madler 
thinks he has detected a very considerable ellipticity in the form 
of the planet, and makes the ratio of the equatorial to the polar 
diameter as ten to nine, the axis being incl'iDed at an angle of 
about 15^ 26' to the <iircle of declination (1843, September 
28). Other astronomei-s, with more powerful telescopes, have 
not succeeded in gaining any certain evidence of an apprecia- 
ble difference in the diameters.* 

The disc of Uranus appears uniformly bright, and of a 
pale color but no appearance of spots or belts has been per- 
ceived. For this reason, the time of axial rotation has not 
been ascertained, though it is probably not very Vv'idely differ- 
ent from that of Jupiter or Saturn. No indications of a ring 
or double ring have been afforded by the powerful telescopes 
of the present day. Sir W. Herschel hinted at the possibility 
of such an appendage, but seems to have had httle confidence 
in the observations which led him to make this suo-o-estion. 

CO 

The planet Uranus is accompanied by several satelhtes 
which require very great optical power to render them visible. 
Sir William Herschel considered he had seen six of these little 
moons, and was able to approximate very closely to the periods 
of two out of this number, which are much more conspicuous 
than the rest. The periodic times of the other four were in- 
ferred, on Kepler's law, from estimated values of their mean 

■^ Mr, 0, Struve has informed me orally that the grand refractor at 
Pulkova affords no indications of ellipticity. 

8 



170 THE SOLAR SYSTEM. 

distances from the centre of tlie primary. The two satelhtes 
with w^hich we are best acquainted are usually denominated 
the second and fourth, the numbers being reckoned in order 
of distance from the planet. 

Sir William Hei-schel states that he had frequently directed 
large telescopes to this remote object, before the year 1787, 
with the view of ascertaining if it were attended by satellites ; 
but the situation of the planet, in a part of the heavens closely 
studded with small stars, led to such continual disappointment, 
that he ascribed his failure to a want of sufficient light, and 
for a time relinquished the attempt. At the commencement 
of the year 1787, Sir William found so great an advantage in 
the introduction of what he termed the front view in his 
powerful reflectors, when applied to the examination of the 
nebulae, that he immediately concluded it would be attended 
with success if applied to the observations of his new planet, 
and accordingly began a close scrutiny of the telescopic stars 
near it on the 11th of January. On this, the very first evening 
that he attacked the planet with his improved means, he saw 
the two larger satellites which we have termed the second and 
fourth ; and a month's observations sufficed not only to con- 
firm the discovery, but to give a very fair idea of the paths 
pursued by these distant bodies. Accordingly, on the 11th of 
February, 1787, he announced his success to the Eoyal Society, 
and assigned the periodic times of the satellites 8-f- days, and 
13-} days respectively. These approximations were j^retty near 
the truth, as the following results obtained from long-continued 
observations will show : — 

Second Satellite. 

D. H. M. s. 
Period, according to Sir W. Herschel's later ob- 
servations, 8 16 56 5*2 

Sir John Herschel, by a comparison of his own 

observations with his father's, . . . 8 16 56 31 '3 



URANUS. 



171 



H. 

16 



56 



s. 
28-55 



8 16 56 24-88 



13 11 8 59-0 



13 


11 


7 12-6 


13 


11 


7 5-92 


13 


11 


7 9-22 


13 


11 


6 55-21 



Dr. Lamont, from measures taken at Munich 
Observatorr, 

Mr. Adams, from the combination of all the ob- 
servations between 1787 and 1848, 

Fourth Satellite. 

Period, according to Sir W. Herschel's latest cal- 
culations, 

Sir John Herschei, by a comparison of observa^ 
tions between 1787 and 1832, 

Dr. Lamont, from Munich Observations, 

The author, by comparison of observations be- 
tween 1787 and 1848, .... 

Mr. Adams, from the whole series of observations. 



Mr. Adams' numbers, which depend on a discussion of all 
the observations of the two Herschels, Dr. Lamont, and Mr. 
Lassell, up to the present time, must, of course, receive the 
preference. 

There is still a great deal of uncertainty with respect to the 
other satellites discovered by Sir W. Herschel at a later period 
than the second and fourth. The observations taken by that 
astronomer between the years 1790 and 1801 sufficiently es- 
tablish the existence of at least four additional satellites ; but 
it is doubtful whether they are definite enough to point out the 
correct times of revolution and mean distances from the planet, 
and hence the difficulty of identif^ring these objects from time 
to time. Sir W. Herschel assigned the following numbers, 
which, however, should be regarded rather as the results of 
calculation than as conveying any exact information respecting 
the true periods and elongations of the satellites, for it is evi- 
dent, from a consideration of the scanty data which Sir Wil- 
liam had to work upon, that he could not determine from them 
the exact elements, nor, indeed, does he affect to speak of his 
conclusions with any degree of confidence in either of his me- 
moirs on the subject : — 



172 THE SOLAR SYSTEM. 



Sat. 


Revolution. 


Mean distance in 




d. h. m. s. 


semi-diameters of Uranus. 


I. . 


5 21 25 20 . 


. 13120 


III. . 


. 10 23 2 47 . 


. 19-845 


V. . 


. 38 1 48 . 


. 45-507 


VI. . 


. 107 16 39 56 . 


. 91008 



These numbers being reckoned in order of distance from 
the primary, it will be remarked that the orbit of the first sup- 
plementary satellite is interior to that of the closest of the 
two with which we are best acquainted ; that the second on 
the list is intermediate, and the two others more distant from 
Uranus than either of the old ones. 

The existence of an interior satellite is inferred from Sir W. 
HerscheFs observations on the 15th and 16th of February, 1798. 
On the 15th a very small star was seen in the line of great- 
est northern elongation, which had moved away on the 16th. 
Another observation of a satelHte supposed to be identical with 
this w^as obtained on the l7th of April, 1801, at lOh. 30m.: 
it was at a great angle in the south preceding quadrant, about 
81°, and at half the distance of the second satellite from the 
planet's centre. On the following night no star was visible in 
the same position. 

The intermediate satellite was detected on the 26th of 
March, 1794, and was steadily seen more than two hours. At 
llh. 24m., Sir W. Herschel noticed that it was much smaller 
than the closest of the two older satellites, and in a line with 
it and the planet : its position was in 59^° in the north follow- 
ing quadrant. On the. following evening it had moved away. 

The existence of the first of the exterior satellites was con- 
sidered to be established by an observation on the 9th of Feb- 
ruary, 1790, v/hen a satellite was remarked at twice the distance 
of the fourth, and in a line with it and the planet ; the angle 
of position being QVb"^ south following. On the 12th, Sir W. 



URANUS. 173 

Herschel observed that the supposed satellite of the 9th was 
not in the place where it was then seen. Other observations 
of this object were made on February 26th, 1792, March 5th, 
1796, and February 11th, 1798. 

The sixth, or most distant satellite, was perceived by Sir 
W. Herschel on February 16th, 1798, at or near its greatest 
southern elongation : it v/as described as excessively faint, and 
the least haze rendered it invisible. On the 18th, it had moved 
away from this place, and had approached nearer to the pri- 
maiy. At llh. 25m. p.m., mean time at Slough, the angle of 
position was 80-9^ south preceding. Two previous observa- 
tions, on February 28th, 1794, and March 28th, 1797, Avere 
supposed to refer to this satellite, but its existence was hardly 
estabhshed till February 1798. 

Sir John Herschel repeatedly observed the second and 
fourth satelhtes between the years 1828 and 1832, but did not 
succeed in recovering any of the supplementary ones. Dr. La- 
mont commenced a course of observations on the two brio-hter 
satellites with the view of determining the mass of the planet 
Uranus with greater accuracy, and on one occasion, in October 
1837, he discovered a very faint object, which he considered to 
be the most distant of the Herschehan satellites, though, for 
want of later observations, he could not speak positively as to 
the identity. Sir John Herschel used his twenty-feet reflector, 
with which his surveys of the heavens have been made. Dr. 
Lam.ont employed the great refracting telescope at the Royal 
Observatory of Munich, which has a clear aperture of ten 
inches, English measure. 

The more recent observations of Mr. Lassell at Liverpool, 
and Mr. Otto Struve at Pulkova, near St. Petersburg, appear 
to decide the existence of at least two satelhtes within the orbit 
of HerschePs second., or the closest of the brighter ones. 
During the latter part of the year 1847, both observers paid 



174 THE SOLAR SYSTEM. 

particular attention to their measures of these satellites, and 
searched carefully for the others. Mr. Lassell repeatedly ob- 
served another one on the northern side of Uranus ; and Mr. 
Otto Struve also detected a third satelhte, which, singularly 
enough, was invariably observed on the southern side of the 
planet. In the w^hole series of observations at Liverpool and 
Pulkova, only one night is common to both, and this was so 
unfavorable at Liverpool, that the faint satellite of Mr. Struve 
might have been easily overlooked. Mr. Lassell's satellite 
would seem to have a periodic time of 2d. 2h. 39m. 36s., 
agreeably to the calculations of the Rev. W. R. Dawes, while 
the observations of Mr. Otto Struve's can only be satisfied by 
a period of 3d. 22h. 8m. 35s. Mr. Lassell noticed a satellite 
on September 27th, 1845, which seems very Hkely to have 
been identical with that recosfnized as'ain by Mr. Otto Struve 
on October 8th, 1847 ; and, in fact, it was by assuming the 
observations to belong to the same object that the above period 
was inferred. It is probable that Mr. LasselPs satellite dis- 
appears (at least to the most powerful telescopes of the present 
day) when it is in the southern portion of its orbit, as Mr. 
Otto Struve's does in the northern^ but this can only be decided 
from further observations. 

On the 6th of November, 1847, Mr. Lassell observed a 
satelhte at about 10'^ distance from the planet, and almost 
precisely opposite to that of the second of Sir W. Herschel's 
list. It is shown by the Rev. W. R. Dawes that this object 
could not have been either Mr. LasselFs or Mr. Struve's satellite, 
but that its orbit is probably intermediate, its greatest distance 
being 15^''. It is true, in this case, we have only a single ob- 
servation to depend upon, and it may seem hazardous to form 
any opinion therefrom ; but the night of November 6th was 
unusually fine, and the planet w^as viewed for more than two 
hours, during which interval this supposed satellite was canned 



URANUS. 1Y5 

• 
along tvitk it. Hence Mr. Dawes thinks it is probable tliat 
there are three satelHtes interior to the second of Sir W. Her- 
schel, at apparent mean distances of 12^', 15' ^ and 18". If 
any one of these be the same as the interior satelhte of that 
astronomer, it would appear most likely to be that detected by 
Mr. Otto Striive, which revolves in 3d. 22h. 8m. 35s. 

From what has been here stated, the reader will easily un- 
derstand that a considerable degree of uncertainty still attaches 
to this question ; not perhaps so much in reference to the nuDi- 
her of satellites as to their relative mean distances from the pri- 
mary. As Uranus is becoming every year more favorably loca 
ted for obseiwation in this hemisphere, it is to be hoped that 
the great telescopes now found in so many observatories will 
clear up all doubts, and place us in possession of something 
like a fair estimate of the number and movements of his atten- 
dants. 

The mass of Uranus has been inferred from observations of 
the two brighter satellites, whose elongations from the primary 
have been repeatedly measured, with this object in view, by Sir 
W. Herschel, Dr. Lamont, Mr. Lassell, and Mr. Otto Struve. 
The recent calculations of Mr. Adams, founded upon their data, 
indicate that the Sun s mass exceeds that of the planet in the 
ratio of about 21,000 to 1, and this result is undoubtedly a 
close approximation to the truth. M. Bouvard had made it as 
17,918 to 1, from the perturbations of other planets by Uranus ; 
but this being considered too large, Dr. Lamont was induced to 
undertake a course of observations in 1837, from which he con- 
cluded the ratio of the masses as 24,605 to 1. It will be re- 
marked that the value assigned by Mr. Adams is intermediate 
to the numbers of M.M. Bouvard and Lamont. 



CHAPTER XIL 

NEPTUNE. 

THE tables of Uranus at present employed in the calculation 
of the planet's apparent positions were composed by M, 
Bouvard of Paris, and published in the year 1821. In the for- 
mation of these tables, M. Bouvard washed to combine the 
ancient observations of Flamsteed, Le Monnier, &c., with the 
whole series between 1Y81 and 1820, and thus produce the 
means of predicting the future j^laces of the planet, from a study 
of its motion through a period of 130 years. But in the course 
of this investigation an unexpected difficulty was encountered. 
M. Bouvard found it impossible to represent the whole of the 
observations, ancient and modern, by one elliptic orbit, even 
after a consideration of the disturbances due to the action of 
Jupiter and Saturn, as indicated by the formulae of Laplace. 
At the time, there appeared to be no satisfactory explanation of 
this discrepancy, and the able astronomer preferred abandoning 
the old observations altogether, and founding his tables on the 
positions from 1/81 and 1820, which could be fairly reconciled 
with an elliptic orbit, with proper allowance for the perturba- 
tions produced by known jDlanets. This curious anomaly excited 
no further remark among^st mathematicians for some few years 
after the publication of the tables; but in the year 1828, the 
Cambridge observations of Uranus showed a very sensible dif- 
ference between the planet's true places and those calculated 
from theory. The circumstance had evidently attracted M. 



NEFTUNE. 177 

Bouvard's attention ; for in a letter written in November, 1834, 
by the Rev. T. Hussej to Mr. Aiiy, then Pkimian Professor at 
Cambridge, it is stated that the French astronomer had been 
led to suspect the existence of an exterior planet to Uranus in 
order to account for the discordances, an idea which had also 
occurred to Dr. Hussey himself. Some correspondence had 
taken place on the subject between Mr. Airy and Mr. Eugene 
Bouvard, nephew of the author of the tables, in the course of 
which the former gentleman expressed an opinion to the effect 
that, if the differences between calculation and observation arose 
from the action of an unseen planet, it would be a matter little 
short of an impossibility ever to ascertain its place in the heav- 
ens. The excessive difficulty of the problem, afterwards so un- 
expectedly solved, will be readily imagined after this deliberate 
ojDinion from one of the highest geometers of the day. 

Early in the year 1843, Mr. Adams, of St. John's College, 
Cambridge, commenced an examination of the theory of Ura- 
nus, and after satisfying himself that the errors of M. Bouvard's 
tables could neither be ascribed to oversight in calculation, or to 
corrections required by the pure elliptic elements of the planet's 
orbit, he directed his attention to the probable effect of a more 
distant planet, and succeeded in obtaining an approximate solu- 
tion of the inverse problem of perturbations^ in which certain 
observed disturbances are given, to find the positions and path 
of the body producing them. In this first solution (which it 
must be remarked was a grand step in the inquiry), Mr. 
Adams assumed that the unseen planet moved round the Sun 
in a circular orbit, at twice the mean distance of Uranus, and 
his results were so satisfactory as to induce him to enter upon the 
subject again, starting from more complete data, and working- 
out the problem without any hypothesis respecting the form of 
the orbit. An application w^as made to the Astronomer Royal 
(Mr. Airy), through Professor Chalhs, for some quantities fur- 



178 THE SOLAR SYSTEM. 

nislied by the Greenwich observations of Uranus, and, in reply, 
the whole of the heliocentric errors in longitude and latitude, 
between 1754 and 1830, w^ere placed at Mr. Adams' service. 
From these data, and the ancient observations of Flamsteed, he 
started afresh, and, in October 1845, communicated to Mr. 
Airy the result of his second, and more complete investigation. 
He remarked that the observed irregularities in the motion of 
Uranus might be explained by supposing the existence of a 
more distant planet, the mass and orbit of Avhich w^ere as fol- 
lows : — 

Mean distance from the Sun, assumed nearly in 

accordance with B ode's law 38"4 

Mean longitude on October 1st, 1845 .... 323-34° 

Longitude of the perihelion 315*55° 

Eccentricity of the orbit 01610 

Mass, that of the Sun being called 1 . . . . 00001656 

The Astronomer Royal replied to Mr. Adams' communica- 
tion on November 5th, 1845, observing that these numbers 
were very satisfactory, and further inquiring whether the as- 
sumed perturbation would explain the error in the distance of 
Uranus from the Sun, wdiicli had become very considerable, and 
was first pointed out by Mr. Airy, in 1836. From some acci- 
dental cause, no immediate answer to this query was sent ; but 
Mr. Airy states, that had he received an affirmative reply, be 
should at once have exerted all tlie influence he might possess, 
either directly or indirectly, through Professor Challis, to pro- 
cure the publication of Mr. Adams' theory. As it happened, it 
w^as not printed until a twelvemonth after this time. 

In the summer of 1845, M. Le Yerrier, the eminent French 
mathematician, turned his attention to the anomalous move- 
ments of Uranus, being entirely ignorant of the researches al- 
ready commenced by Mr. Adams. In the " Comptes Bendus'''* 



NEPTUNE. 179 

of the Institute of Paris for IN'ovember lOth, 1845. appeared a 
most valuable memoir by M, Le Verrier upon the theory of 
Uranus, as regards the perturbations produced by the planets 
Jupiter and Saturn. He determined at the expense of a vast 
amount of labor, the precise effects to be attributed to the action 
of each of these bodies, and after carefully comparing his new 
theory with the observations, ancient as well as modern, he 
announced, as the principal result of his investigation, that the 
anomalies in the motion of Uranus could not be explained, on 
the principles of gravitation, without admitting the existence of 
some extraneous influence. 

On the 1st of June, 1846, M. Le Verrier pubhshed in the 
same periodical his second memoir on the planet, the first part 
of which contained a discussion of nearly all the existing obser- 
vations of Uranus, in reference to the corrected theory of per- 
turbations given in the former paper : the result of this great 
labor was to prove beyond the possibility of doubt that the 
movements of the planet were affected by some external action, 
and M. Le Verrier accordingly proceeds to examine in the sec- 
ond part of his memoir the various explanations of the irregu- 
larities that might be suggested. Could they be due to the 
failure of the law of gra\dtation at the great distance of Uranus ? 
This idea the eminent mathematician rejects as too improbable, 
all previous suspicions of the kind having ultimately tended to 
confirm that law. Could they be owing to the action of a great 
satellite accompanying the planet L^ranus ? In this case the 
discordances should pass through regular variations of magni- 
tude in a certain period, the extent of which would be pretty 
easily determined from a long and continuous series of observa- 
tions. But the errors of the theory followed no such law of 
change. Had a comet at some past time impinged upon Ura- 
nus, and changed its orbital velocity and direction of motion ? 
To this question M. Le Verrier replies that the observations be- 



180 THE SOLAR SYSTEM. 

tween 1781 and 1820 could be very well represented without 
having recourse to any extraneous action, so that the disturb- 
ing force, of whatever kind it might be, had exercised no visible 
influence during that interval. But then the theory which 
would be reconcilable with observations between 1*781 and 
1820 should also be compatible either with the observations 
previous to the year 1781, or subsequent to 1820, yet it had 
been shown that neither the earlier or the later series of posi- 
tions could be brouofht into aoTcement with it. A sino-le colli- 
sion with a comet would not, therefore, explain the anomalous 
movements of Uranus. There remained only the hypothesis 
of an unseen planet of considerable mass, and on this point M. 
Le Yerrier observed it must be situated exterior to the orbit of 
Uranus, or it could not fail to produce some appreciable effect 
upon the motion of Saturn ; whereas nothing of the kind could 
be detected. Consequently, admitting the existence of an ex- 
terior planet, it would be necessary to place it at such a dis- 
tance that the Saturnian system should not be influenced by it 
in any sensible degree, though not so remotely distant as to 
preclude the possibility of its exercising a very powerful attrac- 
tion upon Uranus. M. Le Yerrier, partly guided by Bode's 
empirical law of distances, though without adhering strictly to 
the indications of that law, and further observing that the per- 
turbations in latitude produced by the disturbing body were 
very insignificant, proposes the following question : — " Is it pos- 
sible that the inequalities of Uranus are due to the action of a 
planet situated in the ecliptic, at a mean distance double that 
of Uranus ? If so, where is the planet actually situated, what 
are its mass and the elem.ents of the orbit it describes ?" This 
'intricate problem M. Le Yerrier resolves in his memoir of June, 
1846. I^ow, if we could determine for any tim.e the variation 
due to the action of a planet of unknown mass, we might as- 
certain immediately the direction in which Uranus would be 



NEFTUNE. 181 

attracted, in consequence of the continiions action of the dis- 
turbing body, and hence we shonld also find the position of 
this body amongst the stars. But M. Le Yerrier shoTvs that 
the problem is very tar from presenting itself thus simply. The 
direct determination of the effect of the disturbing planet was 
not possible unless we could ascertain the exact orbit which 
Uranus would describe if uninfluenced by it, and this there are 
no means of discovering unless we are acquainted with the pre- 
cise amount of the perturbations. It wa-s impossible to resolve 
the problem into two distinct heads, the determination of the 
elliptic elements of Uranus, and of the planet to which the ir- 
reo^ularities in the motion of Uranus were referable. The 
method adopted by the eminent mathematician, in his re- 
searches, was to assume the planet located in different parts of 
the ecliptic, and to calculate the amount of alteration which it 
would produce in the longitude of Uranus at each of these dif- 
ferent points. The computations were executed for every tenth 
of a quadrant, or for every ninth degree ; the results showed 
that in one position of the disturbing body its effect upon the 
longitude of Uranus would be excessively great, while in an- 
other position it would vanish entirely, and thus M. Le Yerrier 
was led to that precise part of the heavens vrhere it was neces- 
sary to place the perturbing body in order to represent com- 
pletely the anomalous motions of Uranus. He concluded that 
there was only one region of the ecliptic, where the unseen 
planet could be located, and further, that the observed irregu- 
larities might be perfectly explained, if the existence of a planet 
in that region were admitted, its mean distance from the Sun 
being about double that of Uranus. Taking the 1st of Janu- 
ary, 184Y, as an epoch, M. Le Yerrier announces as the princi- 
pal result of his researches that the heliocentric longitude of the 
disturbing body would be 325^, and this position could hardly 
be in error to the extent of 10^ one wav or the other. In an- 



182 THE SOLAR SYSTEM. 

swei' to a question from Mr. Airy, M. Le Yemer shows that the 
errors in the radii- vectores of Uranus are fully explained by his 
theory, or rather, we should say, disappear altogether on its 
application. About a ^yeek after the receipt of this reply, or on 
the 9th of July, 1846, Mr. Airy wrote to Professor ChaUis at 
Cambridge, inquiring whether he could undertake the search 
for the disturbing body, the existence of which now appeared to 
be placed beyond doubt. Professor Challis having at command 
one of the largest refracting telescopes in this country, the gift 
of the Duke of Xorthumberland to the University, Mr. Airy 
considered he would possess the means most likely to lead to 
the discovery of the planet if it were faint, as was generally an- 
ticipated by those astronomers who had seen the memoir of M. 
Le Yerrier. In answer to Mr. Airy's inquiry the Professor ex- 
pressed his intention of commencing a strict search at once ; 
the examination to be extended over a part of the heavens 30^ 
long, in the direction of the echptic, and 10^ broad. The 
necessary observations were begun on the 29th of July, and 
continued during August and September. 

In the Comptes Rendus of the 31st of August there ap- 
peared a third memoir on the inequalities of Uranus by M. Le 
Yerrier, which rs truly a most wonderful production. In the 
first investigation the mean distance of the disturbing planet 
was announced to be twice that of Uranus ; in the second it is 
considered one of the unknown elements, and results directly 
from the solution of the equations, from which the position of 
the planet and the other orbital quantities are found. All the 
ancient observations of Flamsteed, Le Monnier, Bradley, and 
Mayer, are combined with a great number of modern positions 
between 1781 and 1845 ; and, after many unsuccessful at- 
tempts, M. Le Yerrier finally completed the solution of the 
problem which he had propounded in June, 1846, and gave 
the following elements of the orbit of the latent planet : — 



NEPTUNE, 183 



Mean distance from the Sun or semi-axis major 
Duration of a sidereal revolution 

Eccentricity 

Longitude of the perihelion 

Mean longitude on the 1st of January 1847 

The most probable value of the mass 



36-154 

217-387 yrs. 

0-10761 

284° 45' 

318° 47 

l--9300th 



From these numbers the true position of the planet at the com- 
mencement of the year 1847 was found to be 326^ 32\ Hav- 
ing given these important results, M. Le Yerrier proceeds to 
limit the space over which the search for the suspected planet 
should be extended, to make sure of including the true posi- 
tion. The limits depended on the possible variations of certain 
quantities on which the elements of the orbit were based, and 
are stated to be 321° and 335° of heliocentric longitude. But 
M. Le Yerrier expressly mentioned that he considered the posi- 
tions remote from 326^ 32^ as possessing little probability, and 
advised observers to begin their search for the latent body at 
the point immediately resulting from the solution of the prob- 
lem, extending it on each direction as might be found necessary. 
He further gave it as his opinion that the planet would present 
a disc, of about three seconds diameter, or sufficiently large to 
be readily detected with some of the larger telescopes employed 
in observatories. Throughout the whole of this memoir M. Le 
Yerrier speaks most confidently of the result of his prediction : 
he had pointed out to astronomers the only way in which the 
anomalous movements of Uranus could be explained ; he had 
solved all the mathematical difficulties attending it, and finally 
published to the world the position and appearance of the latent 
planet in the heavens, thus leanng little to be accomplished in 
its actual discovery. 

On the 2d of September, 1846, Mr. Adams addressed a let- 
ter to Mr. Airy (who happened to be absent from England), 
giving a further account of his researches on the irregularities 



184 -^iiE SOLAR SYSTEM. 

of Uranus, aud the results of another solution of the inverse 
problem of perturbations in respect to these observed anomalies. 
In the first attempt the mean distance of the disturbing body 
was supposed to be double that of Uranus ; in this new inves- 
tigation it was somewhat diminished, and the agreement be- 
tween theory and observation was found to be more satisfactory 
than before. Mr. Adams then shows from calculations that the 
errors in the distances of Uranus from the Sun, pointed out by 
Mr. Airy, were destroyed or nearly so by admitting the exist- 
ence of a planet with the elements he had assigned. These ele- 
ments on the second hypothesis as to mean distance were as 
follows : — 

Mean longitude of planet^ 1st October, 1S46 . 323° 2' 

Longitude of Perihelion 299-11 

Eccentricity 0-12062 

Mass (tliat of the Sun being called 1) . . . 0-00015003 

The ratio of the mean distance of Uranus to that of the disturl> 
ing planet is considered to be as 0*515 to 1. 

In conclusion, Mr. Adams states that he was '' employed in 
discussing the eiTors in latitude, with the view of obtaining an 
approximate value of the inclination and position of the node 
of the new planet's orbit ;" but he expressed doubts as to the 
probability of any results to be derived from them in conse- 
quence of their being very small. A rough calculation made 
some time before had indicated that the line of nodes would 
fall at about 300° and 120^ of heliocentric longitude, the for- 
mer being the place of the ascending node ; and, further, that 
the plane of the orbit of the new planet would be rather largely 
inclined to that of the ecliptic. 

On the evening of the 23d of September, 1846, Dr. GalJe, 
one of the astronomers of the Royal Observatory at Berlin, re- 
ceived a letter from M. Le Yerrier, containing the latest results 



yEPTuyr. 185 

of Ills analysis, and sti'onglr urging him to employ the great 
telescope at his command in a search for the planet. It so hap- 
pened that the Berhn Academical Chart for the 21st hour of 
right ascension had been completed by Dr. Bremicker, and as 
the map contained every star to the 9-10 magnitude inclusive, 
which was visible within 15^ of the equator, north and south, 
and between 315^ and 330^ of right ascension, the region of 
the heavens to be examined on M. Le Verrier's recommenda- 
tion was included upon it, and nothing but a comparison of the 
map with the sky was required to detect the planet, if it existed 
in the predicted position, and equalled in brightness a star of 
between the 9th and 10th magnitudes. Dr. Galle. therefore, 
took advantage of a fine evening on the same day that M. Le 
YeiTier's letter arrived, and very soon discovered an object re- 
sembhng a star of the Sth magnitude, near the place indicated 
by theory as that of the distiu'bing planet. This object was 
not marked upon Dr. Bremicker's map, and observations were 
therefore commenced at once, with the view of detecting any 
chraige of place. After about three hoius, it appeared that the 
right ascension had somewhat diminished, though the alteration 
was hardly sufficient to justify the immediate announcement of 
a planetary discovery. But the following evening, ai eight 
o'clock, the object had retrograded more than four seconds of 
time, and there remained no further doubt of its being a planet ; 
noi', considering the proximity to M. Le Verrier's place, could 
there be any hesitation in p^ronouneiug it the very body which 
had caused the irregularities in the movements of Uranus. On 
the 25th, Dr. Galle consequently wrote to M. Le Vrriier, in- 
forming him of his discovery of the latent planet, and stating 
the results of some measures of its diameter by himself and 
Professor Encke, which assigned about 2^ seconds of space, 
thus confirming, in the most ren:iarkable manner, the predic- 
tions published by M. Le Verrier on the 31st of Augtist. The 



186 THE SOLAR SYSTEM. 

observed longitude of the planet on the 23d of September, at 
midnight, was 325^ 52*8', and the diurnal motion in longitude 
'74^', while the numbers computed from the theory were 324^ 
58' and 69''. Thus the error of prediction was less than 1^ in 
the geocentric longitude, and the close accordance of the diur- 
nal motions showed that the distance AI. Le Yerrier had given 
could not be very far wrong. The news of this grand discovery 
soon spread throughout Europe : it was known in England on 
the 30th of September, and about the same day in Paris ; but 
M. Le Verrier does not appear to have received any intimation 
till some days afterwards that our countryman had employed 
himself in similar researches to those, the success of which now 
astonished the astronomers of Europe. The investigations of 
the two gentlemen were consequently entirely independent of 
each other ; both had remarked the apparent errors in the ex- 
isting theory of Uranus, and sought to explain them on the 
same assumption, but the direct discovery, on the 23d of Sep- 
tember, of the planet which thus gave evidence of its existencp, 
was owing to the letter of M. Le Verrier to Dr. Galle. 

We have already stated that Professor Challis commenced 
a search for the planet on the 29th of July, 1846. At this 
time the publication of the Berlin Academical Chart for hour 
xxi. was unknown in England, and the Professor was therefore 
under the necessity of forming his own map, which was to de- 
pend on observations taken in the following manner : — Em- 
ploying the great Northumberland telescope erected in the 
grounds of Cambridge Observatory, the positions of all stars 
to the eleventh magnitude, inclusive, that could be conveniently 
taken as they passed through the field of the telesco23e, were 
first noted down ; the magnifying power used gave a breadth 
of field about 9'. In some parts of the heavens, where the 
stars existed in great numbers, some few were necessarily 
passed over; but as it was important to secure the exact 



NEFTUNE. 187 

positions of every star to the eleventh magnitude, the same 
zone was gone over a second time, the instrument being used 
on another method, which allow^ed more time for recording the 
places of the stars. The observations made on the second oc- 
casion being intended to include all stars observed in the first 
sweep, the planet w^ould be readily detected if any star noted 
down in the first examination had altered its position. But as 
many new stars might be observed the second time. Professor 
Challis proposed going over the zone once more, to make quite 
sure of the planet's discovery, if it really existed in the pre- 
scribed region of the sky. Observations w^ere taken on July 
30 and on August 4 and 12 ; the zone examined on the latter 
day being the same that was observed on the 30th of July. 
A partial comparison of the resuHs on those two days showed 
that the plan of observation was effectual, and the search was 
continued till the 29th of September ; but the further com- 
parison of the observations was deferred until the close of the 
season. Professor ChaUis little suspecting, as he has since stated, 
that " the indications of theory were accurate enough to give 
a chance of discovery in so short a time." On the 29th of 
September, the second memoir of M. Le Yerrier came under 
his notice ; and struck with the manner in which the French 
mathematician limited the region to be examined, and with his 
recommendation to endeavor to detect the planet by its disc. 
Professor ChalKs changed his plan of observation on the eve- 
ning of the same day, and out of 300 stars, singled out one 
which appeared to him to have an appreciable diameter, and 
was noted down for a second scrutiny on the next ^\\q night. 
On the 1st of October the news of Dr. Galle's discovery reached 
Cambridge. Professor Challis had recorded the places of 
3150 stars, and was making preparations for mapping them; 
but he was not aw^are at the time whether the planet was 
amongst them or not. Soon afterwards, on continuing the 



188 THE SOLAR SYSTEM. 

comparison of the observations of July 30 and August 12, it 
was found that a star of th.e eighth magnitude in the series of 
August 12 was \yanting in the zone of July 30, and, ac- 
cording to the j^rinciple of the search, it should be the planet. 
This was really the case ; it had advanced into the zone during 
the interval between July 30 and August 12. The former 
comparison had been extended only to the star No. 39, whereas 
the planet was !N'o. 49. Thus an opportunity of announcing 
its discovery was lost. A further discussion of the observations 
showed that the planet had been observed also on August 4 ; 
so that two early places had been secured; and on carrying 
forward the positions from September 23d to 29th, Professor 
Challis ascertained that the object he had singled out on the 
latter evening, as presenting a measurable disc, was no other 
than the planet of which he was in search. 

Mr. Adams communicated the results of his calculations 
to the Royal Astronomical Society in Xovember, and they 
were printed as a Supplement to the Xautical Almanac for 
1851, in December 1846. M. Le Terrier gave his analytical 
computations in detail, in an appendix to the Connalssance des 
Temi:>s^ but the principal conclusions had been published, as 
we have seen, some months before the planet was detected by 
the telescope. 

A good deal of discussion took place with regard to a name 
for the newly-discovered body. Dr. Galle suggested Janus^ 
which M. Le Terrier opposed, as being too significative : and 
after other appellations had been j^roposed (including the name 
of the illustrious mathematician whose recondite researches had 
led to the actual discovery of the planet at Berlin), astronomers 
generally called it Neptune^ the name first mentioned by the 
BureaiL des Longitudes^ and approved by M. Le Terrier him- 
self. 

Such is the history of this most brilliant discovery, the 



grandest of which astronomy can boast, and one that is des- 
tined to a perpetual record in the annals of science — an aston- 
ishing proof of the power of human intellect. 

It is very possible that there may be a considerable number 
of satellites attendant upon Xeptune : but owing to the re- 
moteness of the planet, astronomers have succeeded in observing 
with certainty only one of them, which was discovered by Mr. 
Lassell of Liverpool, with his great reflecting telescope, in Octo- 
ber, 18-46, or very shortly after the first detection of the planet 
by Dr. Galle. It was so faint as to require a sky of the utmost 
purity, and the full aperture of the trlescope. to render it stead- 
ily visible. In the summer of 1S-4T, Mr. Lassell ascertained 
that the periodic time would be about od. 21h., and the great- 
est apparent elongation from Xeptune's centre about IS", or 
little more than six diameters of the primary. He also discov- 
ered that the satellite is much brioht'^r when it 'precedes than 
when it follows the planet in right ascension, a phenomenon 
which obtains in the Saturnian system, and seems to indicate 
that the time of rotation of the satellite upon its axis is equal to 
the periodic revolution roimd Xeptime, as in the case of our 
Moon. Mr. Otto Struve found the satellite at the Central Rus- 
sian Observatory of Pulkova, on the 11th of September, 1847, 
and in the following month it was discerned by Professor Bond 
of Cambridge, U. S., with a telescope of the same dimensions 
as that of Ptilkova. The American astronomer states that he 
has gained pretty strong evidence of the existence of another 
satellite, fainter and more distant from the primary than Mr. 
Lassell's, but never having succeeded "in procuring consecutive 
observations, the reality of this second discovery is not fidly 
confirmed. 

A discussion of all the observations of the satellite up to the 
end of 1848, shows that the orbit is inclined at an angle of 
about 30^ io the plane of the ecliptic, which it intersects in 



190 THE SOLAR SYSTEM, 

120° and 300° of locgitude. The apparent semi-axis major, as 
seen at the distance 30, appears to be IG'Td", so that the real 
mean distance of the satellite from Neptune is 232,000 miles, 
or not very different from the interval which separates the Moon 
from the Earth. The time of a sidereal revolution of the satel- 
lite is 5d. 21h. Om. l7s., according to Professor Bond, or ex- 
actly od. 21h., agi'eeably to an investigation by the author. 
Viewed from the Earth, the orbit appears an ellipse, with the 
longer axis three times the breadth of the lesser one ; but the 
true path of the satellite is not far from circular, and the ellip- 
ticity of the apparent orbit is consequently owing to the small 
inclination of its plane to our line of vision. 

The satellite is estimated to be equal in brightness to a star 
of the fourteenth magnitude. 

The mass of the planet is not yet accurately known. The 
excessive difficulty attending observations of the satellite, and 
the manifest discordances between the measured distances of 
different observers, tend to throw some degree of uncertainty 
upon any conclusions we may deduce from them. Still, there 
is no doubt that we have already apirroxhnQted to the correct 
value of this important element. The theoretical method has 
hardly received rigorous application at present, but the observa- 
tions of the satellite furnish us with the following results : — 

Mr. Otto Struve, from the Pulkova measures, 1-14494. 

Professor Peirce, from the observations by Messrs. Lassell 
and Bond, 1-18780. 

Professor Bond, from his own observations, IS 47-4 S, 
1-19400. 

The author, by a combination of oil the measures, 1-17900. 

At present it appears probable that the mass is somewhat 
larger than in the case of Uranus, and perhaps we are justi- 
fied in stating that it can liardly be greater than 1-15000, nor 
smaller than 1-20000. Bv assuminor that the mass of the 



NEPTVyE. 191 

Sun exceeds that of Xeptune 18,000 times, no great error can 
be incuiTed. 

Mr. Lassell, with his twenty-feet reflector, and Professor 
ChalHs, with the great Xorthumberland telescope, have at va- 
rious times suspected traces of a ring similar to that surround- 
ing the planet Saturn, and at present seen nearly edgeways. 
Professor Bond, of Cambridge, United States, who has under 
his direction one of the largest refracting telescopes in the world, 
announces that he has repeatedly seen some kind of luminous 
appendage to the planet, similar to what might be supposed to 
be the appearance of a thin flat ring, but he does not profess to 
say positively whether the phenomenon is really due to this 
cause, or whether it be owing to close satellites, which very 
probably exist, or to some optical illusion. It is understood that 
the astronomers of Pulkova, in Russia, who are in possession of 
an instrument precisely similar to that of Cambridge, United 
States, have not yet succeeded in observing any appearance 
such as would lead to the suspicion of a ring. The question 
will most likely remain undecided until the planet rises in decli- 
nation in the coui-se of a few years' time, so as to allow the 
gigantic reflecting telescopes to bear upon it advantageously. 
AVe find Mr. LasselFs present opinion to be less in favor of the 
existence of a ring than formerly. 

After the discovery of Xeptune, it became a matter of im- 
portance to ascertain if any observation of the planet existed in 
the catalogues of stars formed in past times. Professor Bessel 
and M. Lalande have determined the positions of a great num- 
ber of stars in the northern heavens, but at the time the former 
astronomer was occupied with his observations, Xeptune was 
always south of his hmit of declination ( — lo^), and conse- 
quently could not have been included in any of the zones ob- 
served at Konigsberg. Lalande, on the contrary, might have 
seen the planet, and, in fact, did so on two occasions, May 8 and 



192 THE SOLAR SYSTEM. 

10, 1795, as \yas discovered almost simultaneously by Dr. 
Petersen, of Altona, and Mr. Sears C. Walker, of Philadeli^hia. 
These observations, combined with the recent ones, have enabled 
astronomers to approximate much more closely to the true ele- 
ments of the planets, than they could reasonably have expected 
to do from modern observations only. The most exact deter- 
mination of the orbit is due to the American astronomer just 
named, who has devoted much of his time and attention to 
the subject. The planet's mean distance from the Sun is 
2,862,457,000 miles; the eccentricity is comparatively small, 
and produces a variation in the length of the radius-vector of 
not more than 49,940,000 miles. The sidereal time of revolu- 
tion is 60126*71 days, or rather more than 164-J- years, which 
is very nearly double the period of Uranus. The planet is near- 
est to the Sun, or in perihelion when its heliocentric longitude is 
in 47' 15', and traverses the plane of the echptic at the ascend- 
ing node in longitude 130^ 7\ the orbit being inclined to the 
Earth's path at an angle of 1° 4V\ 

The author has lately found three observations of Xeptune 
as a star, by Dr. Lamont, at Munich, before its actual recog- 
nition as a planet by Dr. Galle, viz., on the 25th of October, 
1845, and the 7th and 11th of September, 1846. The last 
observations were probably in consequence of a search com- 
menced on the announcement of M. Le Yerrier's remarkable 
results in the ComjJtes Rendus of the French Institute.^ 

* The oi]ly collection of observations in which there appears now a 
probability of discovering an observation of the planet Neptune prior 
to the commencement of the present century, is that of M. Le Monnier, 
whose manuscripts are understood to be preserved at the National Ob- 
servatory of Paris. H would be a great boon to astronomers if these 
valuable observations were printed ; but the expediency of a search 
for any possible positions of Neptune is so great, that it is to be hoped 
we shall soon hear of an examination having been instituted. Many 



NEPTUNE. 193 

The apparent diameter of Neptune which is subject to no 
sensible variation, is about 2*6'', but this diameter reduced to 
the Earth's mean distance from the Sun would subtend an angle 
of IQ'Q" The real diameter of the planet is about 31,000 
miles, or rather less than that of Uranus. These numbere de- 
pend upon careful measurements with some of the most pow- 
ei-ful European telescopes. 

observations of Uranus as a fixed star occur in Le Monnier's journals, 
and Burckhardt thought he had discovered there an observation of the 
plaDet Vesta. 



INDEX. 

Earth: form, diameter, 45; distance from Sun, 46 ; path in the eohp- 
tic, ib. ; obhquity of the ecliptic, ib. ; ancient and modern observa- 
tions of the obhquity, 47 ; precession of equinoxes, 48 ; nutation of 
axis, 49 ; eccentricity of orbit, motion of apsides, variable length of 
the seasons, 50 ; equation of time, apparent and mean time, 52 ; 
sidereal day, sidereal and tropical years, 53 ; anomalistic year, the 
ancient year, 54; table of the length of the year, 55. 

Echpses : meaning of the term, cause of eclipses, 85 ; ecliptical hmits, 
86 ; number of eclipses in a year, 87 ; cycle of eclipses, 88. 

Eclipses of the Moon : their nature, length of Earth's shadow, 103 ; 
phenomena of a total eclipse, ib. ; the Moon not always invisible in 
echpses, 104; ancient eclipses, ib. ; eclipses during the Peloponnesian 
war, 105 ; eclipse observed by Columbus, at Jamaica, ib. 

Eclipses of the Sun : partial, total, or annular, 88 ; total eclipses of 
rare occurrence, 89 ; phenomena observed during a total eclipse, ib. ; 
the corona, 91 ; red flames or prominences, 93 ; stars visible, 96 ; 
effect on the landscape, 96 ; on the animal creation, 97 ; the eclipse 
of July, 1851, the author's observations in Sweden, 98 ; the eclipse 
of Thales, 101 ; of Pericles and Agathocles, 102. 

Equation of time, 52. 

Jupiter: distance, period, and diameter, 132; telescopic appearance 
of surface, ib. ; the belts, 133 ; spots upon the disc, ib. ; axial rota- 
tion, 134; inclination of axis, 135; mass, 142; ancient observation, 
143 ; tables, ib. 

Jupiter's satelUtes : their discovery, 135 ; distances and sidereal revolu- 
tions, 136; degree of brightness, ib. ; magnitudes as seen from Jupi- 
ter, 137; configuratioD, ib. ; rotations on their axis, 138 ; motions 



196 INDEX. 

of the nodes and apsides, 138 ; eclipses, ib. ; velocity of light ascer- 
tained from observations of eclipses, 139; relation between the 
mean motions of the satellites, 140; occultations and transits of 
satellites and their shadows, ib. 



Mare : his distance, 107 ; period, real and apparent diameter, phases, 
ib. ; appearance of his surface, rotation on axis, 108; atmosphere, 
109; mass, ib. ; difference of diameter, 110; parallax of Sun from 
observations of this planet, ib. ; ancient observations. 111 ; occulta- 
tion of Jupiter by Mars, ib. ; tables of the planet, ib. 

Mercury : his period, 23 ; distance, ib. ; axial rotation, eccentricity, 
24 ; phases, real diameter, 26 ; mass, 26 ; transits over the Sun's 
disc, ib. ; phenomena during transits, 29 ; ancient observations, 31 ; 
tables, ib. 

Minor or ultra-zodiacal planets : Bode's law of planetary distances — 
plan of search, 112; Ceres, 113; Pallas, 115; Juno, 117; Vesta, 
118; Astraea, 120; Hebe, 121; Iris, 122; Flora, 124; Metis, 125; 
Hygeia, 126; Parthenope, 127; Victoria, ib. ; Egeria, 128; Irene, 
129; Eunomia, 130 ; nature of these bodies, Olber's theory, ib. 

Moon : her distance, sidereal and tropical revolutions, 56 ; centre of 
gi-avity of Earth and Moon, ib. (note); synodical period, phases, 57 ; 
eccentricity of orbit, 58 ; motion of the nodes and apsides, ib. ; 
diameter, apparent and real, 59 ; mass of the Moon, ib. ; nutation, 
libration, 60; physical libration, 61; the harvest Moon, ib. ; the 
lunar theory, 62 ; evection, ib. ; variation, parallactic inequality, and 
annual equation, 63; secular acceleration, 64; tables, ib. ; reduction 
of Greenwich observations, 65 ; attraction of Venus, 66 (note) ; 
description of surface, ^Q ; selenographic longitudes and latitudes, 
67 ; names of the lunar spots, ib. ; Tycho, a great crater, 68 ; Co- 
pernicus, Kepler, Eratosthenes, 70 ; Manilius and Pico, 71 ; table 
of the altitudes of lunar mountains, ib. ; lunar cavities, 72 ; synop- 
tical table of the breadths of craters, ib. ; the lunar seas — Mare 
Crisium, <fec., 73; poles of the Moon, 74; atmosphere, ib.; occulta- 
tions of stars and planets, 75 ; volcanoes, 76 ; reasons for doubting 
the existence of active volcanoes, 78 ; mass or charts of the surface, 
79 ; models of surface or particular spots, 80 ; appearances in the 
lunar heavens, 81 ; the lunar day, 82; the tides, ib. 



INDEX. 197 

Neptune: history of the discovery, 176-188; name for planet, ib. ; 

mass, 190 ; suspicions of a ring, 191 ; observations previous to 1846, 

Sept. 23, 191 ; distance and period, 192. 
Neptune's satellite: discovery, period, 176; distance from primary 

and apparent brightness, 177. 
Nutation of earth's axis, 49. 

Obliquity of ecliptic, 46. 

Precession of equinoxes, 48. 

Saturn : his period and distance. 144 ; figure ellipticity, apparent and 
real diameters, ib. ; belts and spots upon his surface, 145 ; time of 
axial rotation, 146 ; atmosphere, ib. ; ancient observations, 164. 

Saturn's ring : discovery, 147 ; position with respect to planet, 148 ; 
phenomena, ib. ; divisions in the ring, 150 ; the dark and newly-dis- 
covered interior ring, 153 ; measures of this new ring, 154 ; dimen- 
sions of the rings, 155 ; thickness of rings, ib. 

Saturn's satellites: Sir John Herschel's names, 155 ; Titan, 156 ; Jape- 
tus, ib. ; Rhea, 157; Dione, 158; Tethys, ib. ; Enceladus, 159; Mi- 
mas, 160; Hyperion, 162; diameters of satellites, 163; eclipses, ib. ; 
new investigation, 164. 

Sun: his distance, 11; diameter and telescopic appearance, spots on 
his disc, 12 ; rotation on his axis, 16 ; discoverers of the solar spots, 
18 ; nature of the spots, 19. 

Tides, 82. 

Time, mean and apparent, 52. 
Transit of Mercury, 26. 
Transit of Venus, 39. 

Uranus : discovery, 165 ; early calculations, 166 ; name of planet, 167; 

old observations of the planet, 168 ; sidereal revolution and distance 

from sun, ib. ; apparent and real diameters, 169; appearance of 

planet, ib. ; mass, 175. 
Uranus, satellites of, Sbr W. Herschel's discoveries, 169 ; table of 

second and fourth satellite, 170 ; the other satellites seen by Sir W. 

Herschel, ib. ; observations of Messrs. Lassell and O. Struve, 173 ; 

present state of our knowledge respecting the satellites, 175. 



198 INDEX. 

Venus: revolution, distance, diameter, 83; axial rotation, 34; spots 
upon her surface, ib. ; mountains, S5 ; seen in the daytime, ib. 
(note) ; supposed satellite, 37 ; Lambert's theory of the supposed 
satellite, 38 (note) ; mass, ib. ; transits over the Sun's disc, 39 ; 
table of ancient transits, 42 (note) ; tables of Yenus, 43 ; ancient ob 
servations, 44 ; occultations of stars and planets by Yenus, ib. 

Year, anomalistic, sidereal, and tropical, 53. 

Zodiacal light, 20. 



,** "N 

§ 1 



I 
It 



THE 

SOLAR SYSTEM: 

i 



A DESCRIPTIVE TREATISE UPON 



THE SUN, MOON, AND PLANETS, 

INCLUDING 

^u Sltrntittt nf all tijf %mni iknnram 




BY 

J. RUSSELL HIID, 

FOREIGN SECRETARY OF THE ROYAL ASTRONOMICAL SOCIETY OP 
LONDON, ETC. 



il 



i 

NEW-TORK: 
GEO. P. PUTNAM, 10 PARK PLACE. 

1852. 



Solar 
System. 



'UTSTAM-'S 
POPULAE 
SCIENCE. 



> 

13/ 



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IX.— A BOOK FOR A CORNER. By Leigh Huitt. First Series.— 

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Under this title a series of attractive popular Treatises, by able and competent writ» 
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HI.— THE WORLD AND ITS WORKSHOPS. 

Further announcements in the next Number. 



I 



k\ 





4 



POPULAR NEW WORKS OF FICTION, 

RECENTLY PUBLISHED BY 

GEOSGE P. PUTMAM, 10 Park Place. 



W. STAEBUOKIIATO.—TRE BERBER; or, The Mountaineer of the 
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ROliA'NGE DUST, from the Historic Flacer. 



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his former popular romances." — Gincinnati Oa^ette. 

■ KALOOLAH; or, Jonrneyings to the Djebel 

Kumri. An Autobiography of Jona. Eomer. Edited by W. S. Mayo. New 
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GEO. F. PXTTHAM'S 
RECENT PUBLICATIONS, 



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ITNIFOBM WITH THE ABOVE, 

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^ impt fit ixmt\ Stkks. 

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|l futkep ^ku)i ; 

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IN SIX VOLUMES — Comprising the following — each sold separately, 

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midxy .""—Bisti'ict School Journal. 

•-•-• 

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Being a Chronological, Alphabetical Eecord of more than a Million of Facts, Pohti- 
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consxilted by the merchant, the scholar, the traveller, and the general Te&d.ev.''—Rb?7ie Journal. 

GEO. P. PUTiVAM, Pnblisher.^ew.iork*. ^ 



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